Add support for GitHub as OAuth provider

[VMelnalksnis]

Fixes #451

[VMelnalksnis]

By using AspNet.Security.OAuth.GitHub the setup is quite simple, however now calls to API without any authentication details are redirected to github for sign in due to GitHubAuthenticationHandler now being present, which causes a Challenge to be returned instead of Forbidden. I guess this also would happen if I had correctly (?) added OIDC support, so probably have to finally dig in to the identity, authentication/authorization documentation.

[codecov]

Codecov Report

Merging #706 (2c7e058) into master (c9708c0) will decrease coverage by 0.16%.
The diff coverage is 45.16%.

@@            Coverage Diff             @@
##           master     #706      +/-   ##
==========================================
- Coverage   72.38%   72.22%   -0.16%     
==========================================
  Files         218      219       +1     
  Lines        6275     6301      +26     
  Branches      407      411       +4     
==========================================
+ Hits         4542     4551       +9     
- Misses       1566     1582      +16     
- Partials      167      168       +1     

Impacted Files	Coverage Δ
...bApi/Configuration/Options/OAuthProviderOptions.cs	0.00% <0.00%> (ø)
...omeshade.WebApi/Configuration/AuthConfiguration.cs	82.75% <48.27%> (-8.98%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[VMelnalksnis]

Could not get swagger to work with oauth2/oidc, however it is possible to simply log in and then use swagger with cookie authentication.