Add rule to build python version of protos

[iche033]

🎉 New feature

Depends on gazebosim/gz-bazel#77

Summary

Add rule in bazel build file to build python version of proto files

Test it

Setup up gz-bazel worksspace, update to update_rules_python_version branch (see gazebosim/gz-bazel#77) and build:

bazel build //msgs/...

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.21%. Comparing base (1939fb0) to head (d8a094b).
Report is 14 commits behind head on gz-msgs10.

Additional details and impacted files

@@              Coverage Diff              @@
##           gz-msgs10     #474      +/-   ##
=============================================
+ Coverage      98.19%   98.21%   +0.01%     
=============================================
  Files             27       27              
  Lines           1166     1174       +8     
=============================================
+ Hits            1145     1153       +8     
  Misses            21       21              

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