feat: Add jwks fetch from idp

[mzaniolo]

Closes: #46

This implementation is not caching the jwks. If we expect many request to be validated we should add a cache to it.

[codecov]

Codecov Report

Attention: Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.90%. Comparing base (2fca463) to head (fa1f012).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
synapse_token_authenticator/config.py	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
+ Coverage   80.63%   80.90%   +0.26%     
==========================================
  Files          17       17              
  Lines        1420     1440      +20     
  Branches      158      160       +2     
==========================================
+ Hits         1145     1165      +20     
+ Misses        204      203       -1     
- Partials       71       72       +1     

Files with missing lines	Coverage Δ
synapse_token_authenticator/token_authenticator.py	71.87% <100.00%> (+0.44%)	⬆️
tests/__init__.py	86.48% <100.00%> (ø)
tests/test_oauth.py	100.00% <100.00%> (ø)
synapse_token_authenticator/config.py	66.21% <50.00%> (+0.46%)	⬆️

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