Automatic merge from submit-queue (batch tested with PRs 49114, 48810)

Unify fuzzers and roundtrip tests

- reorganize apimachinery/pkg/api/testing package to avoid circular imports
- make fuzzers modular per apigroup
- add roundtrip test for meta types
- add roundtrip test for a couple of staging apigroups

Follow-up:

- split `pkg/api` tests by apigroup