feat: idp/rp test apps

[dopry]

Implement a minimal testing IDP and RP for maintainers.

There is a single Application configured in the IDP for the RP sample application it used the OIDC Authorization + PKCE flow.

This is a meant to be a starting point for building out further test scenarios.

applies to #1266

Description of the Change

implements a test IDP using DOT and RP as a pre-cursor to developing playwright E2E tests.

Checklist

• [X] PR only contains one change (considered splitting up PR)
• [N/A] unit-test added
• [X] documentation updated
• [X] CHANGELOG.md updated (only for user relevant changes)
• [X] author name in AUTHORS

[codecov[bot]]

Codecov Report

Merging #1332 (50dff93) into master (1c4a997) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1332      +/-   ##
==========================================
+ Coverage   97.44%   97.46%   +0.02%     
==========================================
  Files          32       32              
  Lines        2073     2092      +19     
==========================================
+ Hits         2020     2039      +19     
  Misses         53       53              

Files	Coverage Δ
oauth2_provider/models.py	98.63% <100.00%> (+0.05%)	:arrow_up:
oauth2_provider/oauth2_backends.py	94.64% <100.00%> (+0.09%)	:arrow_up:
oauth2_provider/oauth2_validators.py	94.10% <100.00%> (+0.02%)	:arrow_up:
oauth2_provider/views/application.py	100.00% <ø> (ø)

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[tonial]

I'll look into this PR on Friday.