Support overriding repos files & providing supplemental ones in PRs

[christophebedard]

Relates to #157

Relates to #490

Similar to ci.ros2.org, this allows declaring PR dependencies by providing:

• repos file(s) to override the one(s) defined through the vcs-repo-file-url action input
• supplemental repos file(s) to be used along with the rest

This is done in a PR description, e.g.:

action-ros-ci-repos-override : https://gist.github.com/some-user/some.repos
action-ros-ci-repos-override : https://gist.github.com/some-user/some-other.repos
action-ros-ci-repos-supplemental : https://gist.github.com/some-user/some-supplemental.repos
action-ros-ci-repos-supplemental : file://path/to/some/other/supplemental.repos

but without the space before the colon (otherwise e2e tests for this PR will try to use those fake links).

To provide some feedback, lists of links of detected overrides/supplemental files are printed in the action output, e.g.:

The feature is currently only tested using unit tests since end-to-end tests are a bit harder for this, but I have tested it and it works great.

[codecov]

Codecov Report

Merging #524 (77de8d1) into master (4c63c81) will increase coverage by 6.66%.
The diff coverage is 62.50%.

@@            Coverage Diff             @@
##           master     #524      +/-   ##
==========================================
+ Coverage   35.76%   42.42%   +6.66%     
==========================================
  Files           1        2       +1     
  Lines         151      198      +47     
  Branches       29       36       +7     
==========================================
+ Hits           54       84      +30     
- Misses         97      114      +17     

Impacted Files	Coverage Δ
src/action-ros-ci.ts	32.54% <5.26%> (-3.22%)	⬇️
src/dependencies.ts	100.00% <100.00%> (ø)

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[emersonknapp]

Overall this looks good, but I'd prefer to remove the unrelated formatting