Require redirect_uri if multiple URIs are registered

[dylantack]

Description of the Change

https://datatracker.ietf.org/doc/html/rfc6749#section-3.1.2.3

If multiple redirection URIs have been registered... the client MUST include a redirection URI with the authorization request using the "redirect_uri" request parameter.

This fixes a bug where the first registered URI was used as the default.

Checklist

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

[codecov]

Codecov Report

Merging #981 (7346d2f) into master (4384566) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #981   +/-   ##
=======================================
  Coverage   96.58%   96.59%           
=======================================
  Files          31       31           
  Lines        1729     1733    +4     
=======================================
+ Hits         1670     1674    +4     
  Misses         59       59           

Impacted Files	Coverage Δ
oauth2_provider/models.py	98.69% <100.00%> (+0.01%)	⬆️

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