Add cloud providers via configuration

[grolu]

What this PR does / why we need it:
On top of PR #1434 this PR adds logic to define additional cloud providers via configuration (customCloudProviders).
It is possible to define a custom secret dialog including (simple) validation and help texts.
It is also possible to configure custom input fields that are added to the Infrastructure Details section of the create cluster page.

Supported input types:

• text: Simple plain text input.
• password: Hidden plain text input.
• select: List of values that the user can select.
• select-multiple: Allow to select multiple values from a list.
• json: Object data in JSON format.
• yaml: Object data in YAML input.

Supported validations:

• required
• requiredIf (ref to other field)
• regex
• isValidObject (depends on the type - valid json or a yaml object)

See example configuration below for more details.

A custom cloud provider icon can be defined using an image resource mounted via asset configuration.
This way it is also possible to overwrite icons as well as names for build-in cloud providers (like aws, gpc, etc) and for machine image vendors (ubuntu, gardenlinux, etc.)
It is possible to provide additional required static data for the shoot template of a custom cloud provider via configuration.
For additional required configurable data, input fields for the custom cloud provider can be defined that will appear on the create cluster page.

This PR also adds a configuration option for the available cloud providers (cloudProviderList). This allows to add additional cloud providers as well as configure their order in the Dashboard. It is possible to add cloud providers without additional configuration, the secret dialog and logo will fallback to generic input / icon.

This PR removes the logic that filters cloud profiles without a matching seed in the backend. All cloud profiles show up in the frontend. Cloud profiles without matching seed will show a hint to the user.

Example dashboard configuration for a cloud provider with name custom

cloudProviderList:
  - gcp
  - aws
  - custom
customCloudProviders:
  fooProvider:
    zoned: false
    shoot:
      createFields:
        - key: foo
          path: spec.provider.extra
          hint: Enter Additional data
          label: Additional Information
          type: text
          validators:
            required:
              type: required
        - key: selectFoo
          path: spec.provider
          hint: Select foo value
          label: Select Foo
          type: select
          validators:
            required:
              type: required
          values: 
            cloudprofilePath: data.foo
            key: name
        - key: selectBar
          path: spec.provider
          hint: Select bar value
          label: Select Bar
          type: select
          validators:
            required:
              type: required
          values: 
            - abc
            - def
      specTemplate:
        provider:
          type: custom
          infrastructureConfig:
            apiVersion: custom.provider.extensions.gardener.cloud/v1alpha1
            kind: InfrastructureConfig
            networks:
              vpc:
                cidr: ${workerCIDR}
          controlPlaneConfig:
            apiVersion: custom.provider.extensions.gardener.cloud/v1alpha1
            kind: ControlPlaneConfig
        networking:
          nodes: ${workerCIDR}
          foo: bar
    secret:
      fields:
        - key: namespace
          hint: Enter a valid namespace
          label: Namespace
          type: text
          validators:
            required:
              type: required
            isNamespace:
              type: regex
              value: .+.+--.+
              message: Must be a valid namespace with format a--b
        - key: token 
          hint: Enter a valid token
          label: Token
          type: password
          validators:
            required:
              type: requiredIf
              not:
                - user
                - password
              message: Token is required if no user / password provided
            isToken:
              type: regex
              value: ^.{10}$
              message: Must be a valid token with length 10
        - key: user
          hint: Enter a valid User
          label: User
          type: text
          validators:
            required:
              type: requiredIf
              not:
                - token
              message: User is required if no token provided
        - key: password 
          hint: Enter a password
          label: password
          type: password
          validators:
            required:
              type: requiredIf
              not:
                - token
              message: Password is required if no token provided
        - key: json
          hint: Enter additional Data as JSON
          label: JSON
          type: json
          validators:
            isJSON:
              type: isValidObject
              message: Data must be valid JSON
        - key: yaml
          hint: Enter additional Data as YAML
          label: YAML
          type: yaml
          validators:
            isYAML:
              type: isValidObject
              message: Data must be valid YAML
      help: |
        #Custom Cloud Provider
        <br />
        <img src="/static/vendor-assets/custom.png" width="100px" />
        <br />
        ## Namespace
        Please enter a valid namespace with format a--b
        <br />
        ## Token / Username - Passwod
        Please enter a valid token with exactly 10 chars. Alternatively you can enter username and password.
vendors:
    aws:
      icon: amazon.svg
      name: Amazon
    custom:
      name: Custom CC
      icon: custom.png

Icons can be provided as base64 encoded values when using the helm chart in the frontendConfig.vendorAssets map. Those icons can then be refrenced in the vendor configurations as well as the help html as shown above.
You can also use data: scheme for development. For production it is recommended to provide static assets.

Cloud provider custom rendered in the Dashboard

Which issue(s) this PR fixes:
Fixes #1445 Fixes #554 Fixes #1707 Fixes #207

Special notes for your reviewer:

Release note:

Define order, name and icon of cloud providers in dashboard configuration

Define additional cloud providers in dashboard configuration

Removed the logic that filters cloud profiles without a matching seed in the backend. All cloud profiles will now show up in the frontend. If a cloud profile without a matching seed is selected for a new cluster, a hint will be shown to the user

[gardener-robot]

@grolu You need rebase this pull request with latest master branch. Please check.