Refactor API client auth, add client app settings

[VMelnalksnis]

Fixes #317, fixes #82.

Changes proposed in this pull request:

• Write user modifiable configuration changes to appsettings.user.json
• Handle API client auth using DelegatingHandler

[codecov]

Codecov Report

Merging #332 (b52d18a) into master (d8aa4ad) will decrease coverage by 0.82%.
The diff coverage is 35.42%.

@@            Coverage Diff             @@
##           master     #332      +/-   ##
==========================================
- Coverage   68.55%   67.73%   -0.83%     
==========================================
  Files         232      242      +10     
  Lines        6672     6830     +158     
  Branches      467      484      +17     
==========================================
+ Hits         4574     4626      +52     
- Misses       1943     2043     +100     
- Partials      155      161       +6     

Impacted Files	Coverage Δ
...alonia.Core/Configuration/OidcOptionsExtensions.cs	0.00% <0.00%> (ø)
...onia.Core/DesignTime/DesignTimeGnomeshadeClient.cs	72.63% <0.00%> (ø)
...e.Interfaces.WebApi.Client/GnomeshadeTokenCache.cs	0.00% <0.00%> (ø)
...Interfaces.WebApi.Client/TokenDelegatingHandler.cs	0.00% <0.00%> (ø)
...onia.Core/Configuration/UserConfigurationWriter.cs	15.78% <15.78%> (ø)
...lonia.Core/Authentication/AuthenticationService.cs	50.00% <25.00%> (+30.00%)	⬆️
...a.Core/Configuration/UserConfigurationValidator.cs	26.08% <26.08%> (ø)
...Core/Configuration/ApplicationSettingsViewModel.cs	29.26% <29.26%> (ø)
...de.Interfaces.Avalonia.Core/MainWindowViewModel.cs	17.26% <30.00%> (-4.87%)	⬇️
...erfaces.Avalonia.Core/Configuration/OidcOptions.cs	60.00% <60.00%> (ø)
... and 12 more

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