Make GenericOAuthProvider configurable via env vars

[philippjfr]

Adds support for:

• PANEL_OAUTH_TOKEN_URL
• PANEL_OAUTH_AUTHORIZE_URL
• PANEL_OAUTH_USER_URL
• PANEL_OAUTH_SCOPE
• PANEL_USER_KEY

[codecov]

Codecov Report

Merging #2960 (325abc9) into master (3928b2f) will decrease coverage by 0.01%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master    #2960      +/-   ##
==========================================
- Coverage   83.13%   83.12%   -0.02%     
==========================================
  Files         190      190              
  Lines       24869    24871       +2     
==========================================
- Hits        20676    20673       -3     
- Misses       4193     4198       +5     

Impacted Files	Coverage Δ
panel/auth.py	40.62% <22.22%> (-0.18%)	⬇️
panel/io/reload.py	69.23% <0.00%> (-2.20%)	⬇️
panel/tests/command/test_serve.py	83.54% <0.00%> (-1.46%)	⬇️
panel/pane/idom.py	27.40% <0.00%> (+0.20%)	⬆️

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[TheoMathurin]

Just stumbled upon this. It's very helpful and IMO should be documented in the authentication section. I can submit a PR.

[philippjfr]

That would be great, however I'm currently restructuring the docs entirely (see #4251). Maybe wait until I merge that first.