Bazel rule rewrite

For my use case the existing flatbuffers_cc_library isn't sufficient. For example, it can't accept filegroups as source arguments. I think the best way to handle this would be to write new idiomatic Bazel rules from scratch using providers, custom rules instead of genrules, etc. To start, it would probably make sense to implement these rules in parallel to the existing rules to give people time to migrate.

I'm willing to implement this--would such a PR be accepted or would I be better off creating a separate rules_flatbuffers project? Thanks for any advice!