Add GitLab plugin

[spenserblack]

Resolves #16

If any GitLab expert happens to stumble on this PR, feedback is welcome!

Notes

• Project ID (number) and project path (user[/group...]/repo) should probably both be valid
  • https://gitlab.com/api/v4/projects/gitlab-org%2Fgitlab
  • https://gitlab.com/api/v4/projects/278964
  • https://gitlab.com/api/v4/projects/gitlab-org%2Fgitlab/issues_statistics
• It might be possible to get number of items using x-total-pages header: https://docs.gitlab.com/ee/api/index.html#pagination

[codecov]

Codecov Report

Merging #222 (1b385c9) into main (728ce9c) will decrease coverage by 2.21%.
The diff coverage is 76.10%.

@@            Coverage Diff             @@
##             main     #222      +/-   ##
==========================================
- Coverage   90.51%   88.29%   -2.22%     
==========================================
  Files          16       18       +2     
  Lines         622      735     +113     
==========================================
+ Hits          563      649      +86     
- Misses         59       86      +27     

Flag	Coverage Δ
unix	87.88% <76.10%> (-2.23%)	⬇️
windows	87.88% <76.10%> (-2.23%)	⬇️

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Impacted Files	Coverage Δ
lib/repofetch/gitlab.rb	55.73% <55.73%> (ø)
spec/repofetch/gitlab_spec.rb	100.00% <100.00%> (ø)

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