As a user i want to be able to clone a secret from one cluster to another using PushSecret. At the moment we can only push individual keys, not the whole secret.

Prerequisites:

• this PR makes the PushSecret.spec.data[].secretKey optional: feat: add support for pushing whole Secret resources to a secret store #2288

Once merged we need to implement the provider-specific bits.

This relates to #2166 and the cross-cluster secret push can be implemented with this feature, however do also want to make it work without having to define a template.

AC:

• users are able to clone a secret across clusters using PushSecret
• we have end-to-end tests to cover this scenario