Migrate to Github CI, test rust package compilation during CI

[bergercookie]

Current PR serves two purposes:

• It migrates to the relatively new "Github Actions" CI framework of github
• It adds a toy test package under test/rust-sample-package which during CI we can build using colcon and the colcon-cargo extension. This way we can be sure that the basic functionality of colcon-cargo always works.

P.S. I'm running the tests in a matrix structure so that we can potentially test more combinations of Ubuntu, Python and/or rust packages in the future. Currently the matrix includes only Ubuntu 20.04 and Python 3.8

Here's how a test runs looks: https://github.com/bergercookie/colcon-cargo/runs/1400198382?check_suite_focus=true

[bergercookie]

Current PR addresses issue https://github.com/colcon/colcon-cargo/issues/11.

[esteve]

@bergercookie thanks!