feat(oidc): client credentials grant type

[james-d-elliott]

Adds support for the client credentials grant type.

[authelia]

Artifacts

These changes are published for testing on Buildkite, DockerHub and GitHub Container Registry.

Docker Container

• docker pull authelia/authelia:feat-oidc-grant-type-cc
• docker pull ghcr.io/authelia/authelia:feat-oidc-grant-type-cc

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[codecov]

Codecov Report

Merging #5729 (e539f5c) into master (662dbd3) will increase coverage by 0.08%.
Report is 1 commits behind head on master.
The diff coverage is 90.90%.

❗ Current head e539f5c differs from pull request most recent head 5a1d2d1. Consider uploading reports for the commit 5a1d2d1 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5729      +/-   ##
==========================================
+ Coverage   72.79%   72.88%   +0.08%     
==========================================
  Files         276      276              
  Lines       22880    22885       +5     
  Branches      438      438              
==========================================
+ Hits        16656    16680      +24     
+ Misses       5497     5480      -17     
+ Partials      727      725       -2     

Flag	Coverage Δ
backend	74.22% <90.90%> (+0.14%)	⬆️
frontend	56.17% <ø> (-0.59%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
internal/handlers/handler_oidc_token.go	47.72% <0.00%> (ø)
...rnal/configuration/validator/identity_providers.go	99.35% <100.00%> (+<0.01%)	⬆️
internal/oidc/discovery.go	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes