PushSecret/Kubernetes: allow cloning of Secrets across clusters

[moolen]

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

[pre]

Cloning of Secrets should also include the ability to clone the metadata.labels of these secrets.

It’s typical that Secrets be found with label search by code (in addition to mounting them by name).

Currently PushSecret only copies the data of the Secret - cloning of labels are not supported at all.

[pre]

Now that the dependency #2862 (follow up of #2288) has been merged, are there any other known blockers for being able support cloning of the whole Secret?

[ron1]

See issue #3353