[wip] use action-ros-ci, add codecoverage

[ddengster]

pardon the sparse commit messages, i couldn't figure out a way to test github actions locally

General changes:

• change workflow to use github actions action-ros-ci, which is used by other packages in the ROS community
• introduces the rmf.repos file, allowing you to pull in other external repositories for testing
• testing/coverage on a per-package basis
• codecov bot now reports coverage

Waiting for PR fix for mixins:

• https://github.com/ros-tooling/action-ros-ci/pull/526

todo: fixing every uncrustify issue that each package's tests will trigger when built. will wait for more PRs to go in

references:

https://github.com/ros-controls/ros2_controllers/blob/master/.github/workflows/ci.yml#L44

https://github.com/ros-tooling/action-ros-ci

https://github.com/colcon/colcon-lcov-result

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@28542c3). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #285   +/-   ##
=========================================
  Coverage          ?   32.41%           
=========================================
  Files             ?      644           
  Lines             ?    59880           
  Branches          ?    39792           
=========================================
  Hits              ?    19412           
  Misses            ?     6648           
  Partials          ?    33820           

Flag	Coverage Δ
tests	32.41% <0.00%> (?)

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