Local conformance tests

[mpstefan]

As a contributor to NGINX Kubernetes Gateway
I want to be able to run the Gateway API conformance tests locally
So that I can test my code is conformant before I create a pull request.

Acceptance

• Any engineer that pulls down the NKG project is able to run the Gateway API conformance tests locally.
• The conformance tests utilize a Makefile
  • Builds local environment
  • Installs NKG
  • Runs conformance tests
• Documentation is added to explain how users can run the conformance tests themselves.

[mpstefan]

Questions for discussion:

• Do we run all tests locally by default, including those we do not yet support?
  • Run all by default until we find that this is annoying.
• Are conformance tests versioned? How do we stay up to date?
  • Dependabot should help us remember to do this