VSS: Add key templating for transformed secret data

[jenneron]

We want to implement the use of this operator, however we have a lot of secrets that have keys like db, connection, etc which we need to result in kubernetes secrets with corresponding keys db.json, connection.json to mount in our pods.

Current transformation only allows to hardcode new name for a specific key, but doesn't allow to dynamically change names of all keys in the secret.

This change allows that by adding keyTemplate to secret transformations so source secret data keys can be renamed with a Go template before being written to the destination Kubernetes Secret.

for example:

spec:
  destination:
    create: true
    name: test
    overwrite: false
    transformation:
      keyTemplate: "{{ .Name }}.json"

This makes:
db -> db.json
connection -> connection.json

---

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• I have documented a clear reason for, and description of, the change I am making.

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.

• If applicable, I've documented the impact of any changes to security controls.

  Examples of changes to security controls include using new access control methods, adding or removing logging pipelines, etc.

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