Break Gradle plugin out of pbj-core and pbj-compiler

[rbair23]

The PBJ project has a funky build due to the fact that it builds a plugin, and that plugin is part of the pbj-compiler project. I think we could instead rearrange the project such that:

root
  |
  - pbj-compiler
  - pbj-runtime
  - pbj-gradle-plugin
  - pbj-grpc-helidon
  - tests
    - pbj-integration-tests

The compiler depends on the runtime, the gradle plugin depends on both, and the helidon plugin depends on all three. There would be a single multi-project gradle build, and each of the above would be a different module.