Wire cpp_hdrs through the build system

I have absolutely no idea what I'm doing but I have the desired end behavior, so instead I'll describe:

1. My motivation
2. What I think I did

Please give me feedback on if there's a better way to do this.

Our story starts here: []

As you can see, this is the end CUJ I ended up with, as shown in third_party/absl/functional/BUILD. You can just have that additional file on the Crubity thingy, and Crubit will understand what you want. The thing to test here is

blaze test //third_party/crubit/rs_bindings_from_cc/test/consume_absl:uses_anyinvocable_test

So how did I get here? I initially just tried to wire these additional cpp_hdrs to the target they were on (absl/functional:any_invocable gets any_invocable_crubit_abi.h as a public_hdr), but quickly realized this doesn't work because while they will be exposed to the clang invocation on that target, when it came time to generate bindings for a dependent, the extra cpp_hdrs, like any_invocable_crubit_abi.h, are nowhere to be found in the transitive closure of includes in my_cpp_library_that_uses_any_invocable.h.

This made me think that these headers need to be wired all the way though all the deps, and be considered as public headers on the top-level cc_library that we are running on to ensure they are being included in the _rs_api_impl.cc file. The way I did this is super messy.

I did some digging, and it seems like RustBindingsFromCcInfo is the provider that goes through all the reverse deps of a target, and AdditionalRustSrcsProviderInfo are the "leaf" providers that attach things along the way. So I added additional_cpp_hdrs to RustBindingsFromCcInfo, and cpp_hdrs to AdditionalRustSrcsProviderInfo. Then in _rust_bindings_from_cc_aspect_impl, I coalesced the additional_cpp_hdrs from any deps along with the cpp_hdrs from the AdditionalRustSrcsProviderInfo on this target, if there was one. Then when we go to call generate_and_compile_bindings, I ensure that public_hdrs also includes these extra C++ headers, put I also pass in the extra headers separately so they can be forwarded to the produced RustBindingsFromCcInfo.