197 fetch oauth public key

[bramtweedegolf]

For external OAuth servers, fetch OAuth public keys from well known location

Fixes #197

[codecov]

Codecov Report

Attention: Patch coverage is 49.50000% with 101 lines in your changes missing coverage. Please review.

Project coverage is 80.79%. Comparing base (b4d17be) to head (82d45de).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
openleadr-vtn/src/jwt.rs	14.89%	80 Missing ⚠️
openleadr-vtn/src/state.rs	87.14%	9 Missing ⚠️
openleadr-client/src/bin/cli.rs	0.00%	5 Missing ⚠️
openleadr-client/src/error.rs	0.00%	5 Missing ⚠️
openleadr-client/src/bin/everest.rs	0.00%	1 Missing ⚠️
openleadr-vtn/src/main.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #214      +/-   ##
==========================================
- Coverage   82.16%   80.79%   -1.37%     
==========================================
  Files          42       42              
  Lines        5690     5791     +101     
==========================================
+ Hits         4675     4679       +4     
- Misses       1015     1112      +97     

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[bramtweedegolf]

A very simple setup for testing with a mock extenal oauth server can be found here:

https://github.com/bramtweedegolf/oauth-mock-setup/blob/main/index.js

Maybe we can add this to a README of documentation somewhere?