Add generator-(function) for existing secrets

[MatteoVoges]

Describe the feature
When working with refs in kapitan it could be useful to initialize kapitan refs without a generator function like random:str.
So I thought of a function exists, that signalizes kapitan, that the secret / ref already exists in the secret engine/ backend.

This might only apply to some secret engines like vaultkv engine, that has writing enabled when #958 gets merged.

The usecase would be, that if I have my secrets already stored in my vault, then I just want to initialize the kapitan-ref-files, but at the moment I have to overwrite it (with kapitan refs or with random:str). So now I just tell kapitan, that the secret already exists and kapitan fetches the secret from the vault and then creates the reffile.

On the other hand we could have a generator function, that takes the secret value as input like ?{backend:my/path||value:mysecret}. This would allow us to store the secret directly without the need of the kapitan refs command. However this would require that the developer removes the secret after creation, so that no secret value is plain in the inventory.

[MatteoVoges]

Update: This is already implemented on nexenio-dev but will be integrated in the future.