enable gRPC spoofing

[takakonishimura]

What does this PR do?

• [x] Adds new functionality
• [ ] Alters existing functionality
• [ ] Fixes a bug
• [ ] Improves documentation or testing

Please briefly describe your changes as well as the motivation behind them:

Instead of limiting gRPC application level overrides to returning &emptypb.Empty{}, we now accept a SpoofFunction which you can configure to read the entire request structure and return custom responses to your gRPC requests.

Code Quality Checklist

- [ ] The documentation is up to date. // TODO

• [x] My code is sufficiently commented and passes continuous integration checks.
• [x] I have signed my commit (see Contributing Docs).

Testing

• [x] I leveraged continuous integration testing
  • [x] by depending on existing unit tests or end-to-end tests.
  • [ ] by adding new unit tests or end-to-end tests.
• [x] I manually tested the following steps:
  • Will document before PR is switched to Ready for Review
  • [x] locally.
  • [ ] as a canary deployment to a cluster.