Make C++ RN Modules explicitly specify that they are C++ modules

Summary:
Currently `ReactModuleSpecProcessor` looks at each of the modules, and sees if the module inherits from `CxxModuleWrapper` to see if it is a C++ module or not. With this change, we now require C++ modules to explicitly specify that they are C++ modules, instead of doing annotation processing magic.

Also note that annotation processor do not work with interfaces but with classes only, so this also removes the dependency of the annotation processor from the bridge buck target.

Reviewed By: achen1

Differential Revision: D9597352

fbshipit-source-id: fd847ac382699d2ab78f7d7c6e2dbd7c60d3f0c4