Use custom scheme for oauth redirect

[VMelnalksnis]

Changes proposed in this pull request:

• Use gnomeshade:// scheme for OAuth redirects instead of http

[codecov]

Codecov Report

Merging #498 (1d5e67d) into master (96047c3) will increase coverage by 0.03%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #498      +/-   ##
==========================================
+ Coverage   66.59%   66.62%   +0.03%     
==========================================
  Files         280      280              
  Lines        8498     8465      -33     
  Branches      550      546       -4     
==========================================
- Hits         5659     5640      -19     
+ Misses       2658     2647      -11     
+ Partials      181      178       -3     

Impacted Files	Coverage Δ
...hade.Avalonia.Core/Authentication/SystemBrowser.cs	0.00% <0.00%> (ø)
...meshade.Avalonia.Core/Configuration/OidcOptions.cs	42.85% <0.00%> (ø)
...Gnomeshade.Avalonia.Core/Authentication/Browser.cs	65.00% <100.00%> (+4.62%)	⬆️

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