add azure oauth v2.0 endpoint support

[j-r77]

Added a (very simple) additional authentication class that uses Azure AD's v2.0 oauth endpoint. This one uses the standard openid, email, and profile scopes instead of Azure-specific ones.

In particular, I couldn't get the default azuread auth to work with accounts that require MFA.

Azure's own docs suggest using V2.0 for all new applications:
https://docs.microsoft.com/en-us/azure/active-directory/azuread-dev/v1-overview

[philippjfr]

Love it, thanks for contributing @j-r77!

[codecov]

Codecov Report

Merging #3224 (1a25386) into master (a068ccb) will decrease coverage by 0.02%.
The diff coverage is 72.22%.

@@            Coverage Diff             @@
##           master    #3224      +/-   ##
==========================================
- Coverage   83.10%   83.08%   -0.03%     
==========================================
  Files         193      193              
  Lines       25731    25747      +16     
==========================================
+ Hits        21385    21393       +8     
- Misses       4346     4354       +8     

Impacted Files	Coverage Δ
panel/auth.py	41.45% <72.22%> (+1.50%)	⬆️
panel/io/reload.py	69.23% <0.00%> (-2.20%)	⬇️
panel/tests/command/test_serve.py	83.54% <0.00%> (-1.46%)	⬇️
panel/tests/pane/test_plotly.py	98.15% <0.00%> (-0.62%)	⬇️
panel/pane/idom.py	27.40% <0.00%> (+0.20%)	⬆️

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