Add credstash backend

[knelasevero]

Describe the solution you'd like
Support credstash as a backend for secrets.

What is the added value?
Having new backends increases chances for adoption and broadens the operator use cases.

We have a fellow external secret solution interested in joining forces. https://github.com/ouzi-dev/credstash-operator

Their main need is a credstash so it would be cool if we could implement this backend.

Would be nice community wise to have this connection.

Give us examples of the outcome

apiVersion: store.externalsecret-operator.container-solutions.com/v1alpha1
kind: SecretStore
metadata:
  name: secretstore-sample
spec:
  controller: staging
  # Sample store types
  #
  # credstash  
  store:
    type: credstash
    auth: 
      secretRef: 
        name: externalsecret-operator-credentials-credstash
        namespace: externalsecret-operator-system
    parameters:
      server: credstash.example.com
# Not sure of what parameters would be needed yet

Observations (Constraints, Context, etc):