Add alternative GH auth: GH app

[ledoyen]

⚠️ TODO

GitHub action check

[github-actions]

Test Results

31 tests   31 ✅  33s ⏱️
14 suites   0 💤
14 files     0 ❌

Results for commit 2b28472.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

Attention: Patch coverage is 31.91489% with 64 lines in your changes missing coverage. Please review.

Project coverage is 33.42%. Comparing base (cb78861) to head (2b28472).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
...it/thirdparty/github/GitHubAuthenticationHolder.kt	20.54%	57 Missing and 1 partial ⚠️
...jo/korekto/toolkit/thirdparty/git/GitRepository.kt	50.00%	3 Missing ⚠️
...n/com/github/lernejo/korekto/toolkit/GradingJob.kt	0.00%	1 Missing ⚠️
...ejo/korekto/toolkit/launcher/GradingJobLauncher.kt	0.00%	1 Missing ⚠️
...o/korekto/toolkit/thirdparty/git/ExerciseCloner.kt	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #365      +/-   ##
============================================
- Coverage     33.95%   33.42%   -0.54%     
+ Complexity      141      140       -1     
============================================
  Files            54       55       +1     
  Lines          2082     2124      +42     
  Branches        254      252       -2     
============================================
+ Hits            707      710       +3     
- Misses         1295     1338      +43     
+ Partials         80       76       -4     

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