Forward JWT token auth to scheme based on issuer (#428)

[VMelnalksnis]

Fixes #428

[codecov]

Codecov Report

Merging #717 (1c47253) into master (b461485) will increase coverage by 0.03%.
The diff coverage is 69.23%.

@@            Coverage Diff             @@
##           master     #717      +/-   ##
==========================================
+ Coverage   72.22%   72.26%   +0.03%     
==========================================
  Files         219      219              
  Lines        6301     6305       +4     
  Branches      411      412       +1     
==========================================
+ Hits         4551     4556       +5     
+ Misses       1582     1580       -2     
- Partials      168      169       +1     

Impacted Files	Coverage Δ
...V1/Controllers/ExternalAuthenticationController.cs	55.81% <33.33%> (-2.73%)	⬇️
...omeshade.WebApi/Configuration/AuthConfiguration.cs	85.03% <80.00%> (+2.27%)	⬆️

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