proto <> ros functionality

[mvukov]

cf. #500 and #605.

Given a, let's say, "public" API in protos (that you could use in backend), this PR allows you to have equivalent msg type to use them in ROS. Plus, adds converters.

Adds support for:

1. Generating ROS msg files from proto files. Limitation is 1 msg per proto file. This way Bazel knows that for 1 proto file there is going to be one msg file. This is also a very good practice anyway.
2. Generating ROS <> proto converter functionality.

This is a Bazel-native solution that (again) leverages Bazel aspects. In other words, Bazel takes care that code generation is properly done through the dependency graph. TBH, didn't look into proto2ros closely, but I saw possible issues in dependency propagation when working with Bazel.

The code is messy, brother Claude and I hacked this up in a short amount of time.

TODOs:

• Add support for enums.
• Add "native" conversions -- e.g. proto timestamp <> ROS timestamp. This will likely require a small runtime library.
• Ignore deprecated proto fields for msg code generation. What about deprecated messages? Can code ROS code generation handle empty files?
• Get the code in decent shape.
• Add proper unit tests for the generators and the generated code.

Support for other languages, such as Rust and Python -> follow-up.

@uhlajs @Skeleton003 Is this something you're looking for? There are no docs yet, you can take a look at ros2/test/protobuf/converter_tests.cc. Any specific functionality you're looking at?

[mvukov]

Also, one item high on my wish list is to add some option annotations to be able to define fixed-size arrays on the ROS side (such that one can use zero-copy transport in ROS on generated msgs).