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.

[jjohannes]

I agree.

Additional note: we should check if integrationTests could be splitup into different kind of tests that are located in src/test (and maybe src/testIntegration) of the different projects instead of having it as a separate Gradle build project. Tests for the plugin can be executed using Gradle's TestKit. This would avoid some complicated setup of the separate build.

settings.gradle.kts should be in the root after this change to align with the structure of other Hashgraph repositories.