Add childResource to OAM definition spec

[resouer]

It has been proved that childResource is a strong need for any abstraction workload (e.g. ContainerizedWorkload), so that Trait controller could know what's the underlying workload object it need to handle with, see: this example. Note that this field is Kubernetes specific for now, but it has been proven to be a generic field required by various implementations.

--- Off Topic ---

That being said, we may also want to add a runtime field to definition object in the future so we can add runtime specific configurations in the definition object? For example:

apiVersion: v1alpha2
kind: TraitDefinition
spec:
  runtimePlatform: Kubernetes
 ...

apiVersion: v1alpha2
kind: TraitDefinition
spec:
  runtimePlatform: ServiceFabric
 ...

apiVersion: v1alpha2
kind: TraitDefinition
spec:
  runtimePlatform: AlibabaCloudFaaS
 ...

[resouer]

/cc @ryanzhang-oss @wonderflow

[ryanzhang-oss]

I think it's better to create a separate issue on the "runtime", it deserves its own issue :)

[wonderflow]

I agree this proposal, what about make runtimePlatform: Kubernetes as default?