CORE-127: enforce scalafmt in github actions

[davidangb]

Followup to #337

That other PR performed a mass reformat of code to meet scalafmt standards.

This PR adds:

• a format check in github actions for PRs
• introduction of .git-blame-ignore-revs, so the mass reformat doesn't show up in git blames

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.27%. Comparing base (bcd8b62) to head (19b212a).
Report is 6 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #338   +/-   ##
========================================
  Coverage    59.27%   59.27%           
========================================
  Files           18       18           
  Lines          636      636           
  Branches        68       68           
========================================
  Hits           377      377           
  Misses         259      259           

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[davidangb]

obsoleted by #337