ruby codegen: support generation of rbs files (#15633)

this introduces support for a new protoc option, `--rbs_out`, which points to a directory where ruby type definition files, defined in the RBS format, are stored.

[rbs](https://github.com/ruby/rbs) is the type signature syntax blessed by the ruby core team, used by static analysis tools such as [steep](https://github.com/soutaro/steep), which integrates with VS Code, the `irb` console (for features such as autocompletion). and [typeprof](https://github.com/ruby/typeprof).

It relies on type definitions written into `.rbs` files.

The `protobuf` library already exposes type definitions in [gem_rbs_collection](https://github.com/ruby/gem_rbs_collection/tree/main/gems/google-protobuf/3.22), which is used to source type definitions for libraries which do not want, or can't maintain type definitions themselves.

(`protobuf` could arguably import these into this repository, lmk if you're interested).

This should fix gaps such as better IDE integration, such as the ones described
[here](https://github.com/protocolbuffers/protobuf/issues/9495).

The plan is to do roughly the same type of integration as was done for `.pyi` annotations, which also write to separate files: add the cli option and the rbs generator.

protobuf classes in ruby rely on dynamic attribution of the base class, which makes what I'm trying to achieve a bit difficult. Ideally the type hierarchy could be specified statically in the ruby source code.

```ruby
class Bar < AbstractMessage
class Bar <
Google::Protobuf::DescriptorPool.generated_pool.lookup("Bar").msgclass
```

```

Closes #15633

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/15633 from HoneyryderChuck:ruby-rbs-integration 2167aa91e5de2999536dd036cb3e7bfddab720fd
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