Add new backend for LinkedIn OpenID Connect

[noxan]

Proposed changes

LinkedIn OAuth2 has been deprecated as of August 2023 and been replace with OpenID connect.
This pull request adds LinkedIn OpenID Connect. It has a few specific adjustments as LinkedIn does not send any "nonce" and therefore does not work with the regular OpenID Connect implementation.

Thanks to #829 for the idea and the hint on the OpenID Connect auth base class.

Types of changes

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• Release (new release request)
• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (PEP8, lint, formatting, renaming, etc)
• Refactoring (no functional changes, no api changes)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
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• Lint and unit tests pass locally with my changes
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[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (ca841a0) 77.11% compared to head (e89c1f3) 77.09%.
Report is 7 commits behind head on master.

❗ Current head e89c1f3 differs from pull request most recent head e5b7568. Consider uploading reports for the commit e5b7568 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #833      +/-   ##
==========================================
- Coverage   77.11%   77.09%   -0.03%     
==========================================
  Files         337      337              
  Lines       10262    10276      +14     
  Branches      690      692       +2     
==========================================
+ Hits         7914     7922       +8     
- Misses       2192     2198       +6     
  Partials      156      156              

Flag	Coverage Δ
unittests	77.09% <60.00%> (-0.03%)	⬇️

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Files	Coverage Δ
social_core/backends/linkedin.py	71.62% <60.00%> (-3.38%)	⬇️

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[noxan]

@nijel sorry I missed your initial comment, resolved your feedback. anything else keeping us from adding this pull request?

[nijel]

Can you please also add tests for this?

[nijel]

...and documentation to https://github.com/python-social-auth/social-docs

[noxan]

haha, sure. i'll look into it :)

[noxan]

tests should be ready, i'm preparing the docs as another pull request and will link it here, once i'm ready.

[nijel]

Merged, thanks for your contribution! Please followup with the documentation PR.