Configure User-Agent string for API client

[VMelnalksnis]

Fixes #387.

[codecov]

Codecov Report

Merging #389 (b281429) into master (b3d3101) will decrease coverage by 0.16%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #389      +/-   ##
==========================================
- Coverage   67.14%   66.97%   -0.17%     
==========================================
  Files         249      250       +1     
  Lines        6866     6883      +17     
  Branches      507      509       +2     
==========================================
  Hits         4610     4610              
- Misses       2094     2111      +17     
  Partials      162      162              

Impacted Files	Coverage Δ
...Core/Configuration/ApplicationSettingsViewModel.cs	29.26% <ø> (ø)
....Core/Configuration/ServiceCollectionExtensions.cs	0.00% <ø> (ø)
...s.Avalonia.Core/Configuration/UserConfiguration.cs	100.00% <ø> (ø)
...hade.Interfaces.WebApi.Client/GnomeshadeOptions.cs	100.00% <ø> (ø)
...faces.WebApi.Client/ServiceCollectionExtensions.cs	0.00% <0.00%> (ø)

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