Re-enable the dependency CI and protobuf CI checks

Migrate these two sdk-platform-java jobs over to the monorepo:
- https://github.com/googleapis/sdk-platform-java/actions/workflows/downstream_protobuf_compatibility_check_nightly.yaml
- https://github.com/googleapis/sdk-platform-java/actions/workflows/dependency_compatibility_test.yaml

Ideally, these should be run against the unit tests in these repos :
1. Showcase (GAPIC Coverage)
2. Handwritten (Veneer Coverage)

To ensure that there are no source incompatibilities.

Stretch:
- See if we can also test against handwritten ITs
- If we can test with all the handwritten libraries, we can provide an official dependency compatibility matrix for the major dependencies that we ship in the SDK

EDIT:
July 23 - Added a new stretch goal