Added support for secrets versioning in Secrets Manager using version stage labels

[furikake]

This commit is to add versioning support as per #175

perhaps we want to support both versionId & versionStage, which makes naming it version abit odd?

[furikake]

I figured most people would be labeling their versions with a staging label instead of relying on a version id (UUID) since it's difficult to work with. Happy to update though.

Do you think it version should be renamed:

    - key: hello-service/credentials
      name: password
      # Version Stage in Secrets Manager
      versionStage: AWSPREVIOUS
      property: password_previous

or nested under version.versionStage

    - key: hello-service/credentials
      name: password
      # Version Stage in Secrets Manager
      version: 
        versionStage: AWSPREVIOUS
      property: password_previous

[Flydiverny]

@furikake I think just renaming it to versionStage is fine :)