[Bazel] Implement `flatbuffers_library` and `cc_flatbuffers_library` build rules

[anton-bobukh]

This PR introduces two modernized Bazel build rules for FlatBuffers:

• flatbuffers_library – the basic language agnostic building block;
• cc_flatbuffers_library – a C++ wrapper for flatbuffers_library targets.

[github-actions[bot]]

This pull request is stale because it has been open 6 months with no activity. Please comment or label not-stale, or this will be closed in 14 days.

[lewissettercaci]

not-stale

[jtdavis777]

@anton-bobukh I think it would be great to add some documentation surrounding this PR. I'm not sure where those docs should live, and am happy to merge these without, but it would be good to think about. Also, I'm totally unfamiliar with bazel -- will these rules be ran as part of the normal CI checks?