Improve SecretTemplate to create secrets from different namespaces

[felipenferri]

Describe the problem/challenge you have
We can create secrets from different Namespaces with SecretTemplate

Describe the solution you'd like
Enable option in metadata to add namespace

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[sathyanarays]

Please provide some more details on current behavior and the expected behavior!

[mamachanko]

@felipenferri the Secret(Export|Import) CRD's responsibility is to facilitate and control cross-namespace usage of Secret resources. You can combine these with SecretTemplate to template a Secret from other Secrets you imported into the current Namespace. To work off of SecretTemplate's example in the docs and assuming that SecretExport exist for password and username, consider:

---
apiVersion: v1
kind: SecretImport
metadata:
  name: password
  namespace: my-ns
fromNamespace: other-ns

---
apiVersion: v1
kind: SecretImport
metadata:
  name: username
  namespace: my-ns
fromNamespace: other-ns

---
apiVersion: secretgen.carvel.dev/v1alpha1
kind: SecretTemplate
metadata:
  name: new-secret
spec:
  #! list of resources to read information off
  inputResources:
  - name: username-secret
    ref:
      apiVersion: v1
      kind: Secret
      name: username
  - name: password-secret
    ref:
      apiVersion: v1
      kind: Secret
      name: password
  #! the template that follows a subset of the Secret API
  template:
    #! data is used for templating in data that *is* base64 encoded, most likely Secrets.
    data:
      password: $(.password-secret.data.password)
      username: $(.username-secret.data.username)