feat: allow hardcoding uid in auth

[kiwiz]

Allows hardcoding a UID in AuthExternal and adds a basic test for the functionality.

Closes #188

[codecov]

Codecov Report

Base: 82.33% // Head: 82.22% // Decreases project coverage by -0.10% ⚠️

Coverage data is based on head (0c6e789) compared to base (b3c7d51).
Patch coverage: 87.50% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #189      +/-   ##
==========================================
- Coverage   82.33%   82.22%   -0.11%     
==========================================
  Files          27       27              
  Lines        3153     3157       +4     
  Branches      651      652       +1     
==========================================
  Hits         2596     2596              
- Misses        342      343       +1     
- Partials      215      218       +3     

Impacted Files	Coverage Δ
src/dbus_fast/__version__.py	0.00% <0.00%> (ø)
src/dbus_fast/auth.py	83.01% <100.00%> (+1.38%)	⬆️
src/dbus_fast/message_bus.py	71.48% <0.00%> (-0.74%)	⬇️

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