Support GitLab Code Quality report format

[Johnnei]

Add support to output GitLab Code Quality report format (Fixes #775).

Note: On multi module projects this will need support from the preferred build tool or trick (cat + jq?) to aggregate the jsons into one json.

The report requires project root relative paths to get correct links in the widgets. To this I used CI_PROJECT_DIR (https://docs.gitlab.com/ee/ci/variables/predefined_variables.html). If this turns out to not be flexible enough then maybe it could still become a config option for the report.

[codecov-commenter]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (9f33cec) 87.07% compared to head (055d6a1) 86.75%.

Files	Patch %	Lines
...l/scapegoat/io/GitlabCodeQualityReportWriter.scala	74.19%	8 Missing ⚠️
...main/scala/com/sksamuel/scapegoat/io/IOUtils.scala	0.00%	1 Missing ⚠️
src/main/scala/com/sksamuel/scapegoat/plugin.scala	75.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #803      +/-   ##
==========================================
- Coverage   87.07%   86.75%   -0.33%     
==========================================
  Files         141      142       +1     
  Lines        1540     1578      +38     
  Branches      268      274       +6     
==========================================
+ Hits         1341     1369      +28     
- Misses        199      209      +10     

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[Johnnei]

@saeltz @mccartney Could you be so kind to review this PR?

I've done some basic tests on a (private) GitLab Ultimate instance to test the various features around Code Quality, and they look fine.

[saeltz]

Thanks for your work!

[saeltz]

Maybe extract the MessageDigest to only have one instance instead of initialising one for every warning?

[Johnnei]

As MessageDigest is stateful and not thread safe, I've moved it to the outer loop so it at least re-uses the instance for an entire report.

[gaeljw]

Thanks @Johnnei 👍