WIP: implement secret filtering by keys

[martin31821]

This PR implements an initial approach on #248, by introducing a new annotation.

For ease of implementation, I used the golang slices package, which depends on golang >= 1.21, so I updated .github and golang to 1.22 in the process.

If you'd like me to finish this, I'd add extra tests and documentation and possibly also implement it for configmaps.