Clarify build time code introspection and code generation options

[greenrobot]

ObjectBox usually depends on build time tooling, e.g. parse source code with annotation in an AST so that we can generate some code that is compiled along with the user's code (entity (type) binding). Also, ObjectBox tracks the data model and IDs via a JSON file. The entity binding and the data model JSON itself should go in new tickets once we know better what we have.

What build time tooling is available in Dart? What can it do?

Links:

• https://dart.dev/tools/build_runner
• https://medium.com/flutter-community/part-2-code-generation-in-dart-annotations-source-gen-and-build-runner-bbceee28697b