Azure Core v2: check in code generated in compile-time

This task should be broken down into multiple pieces:

1. Annotation processor should generate code in location that can be checked into the repo
2. It should be possible to easily turn it off once it becomes more mature
3. The precondition is to have a validation as a part of CI pipeline that
- compiles code (and overrides auto-generated files)
- git diff generated sources
- fails if checked in sources are not identical to the auto-generated ones.
To ensure that checked in code remains consistent with the one produced by the generator.