Add native AoT support for ReDoc

[martincostello]

• Add support for using the ReDoc middleware in native AoT applications.
• Add missing test coverage for ReDoc extensions.
• Fix some code analysis suggestions.
• Fix native AoT test app exception on start-up (see [Bug]: Breaking change in 6.6.1; It now requires you to have the Regex Route Constraint even if you don't use it #2951).

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.44%. Comparing base (9fa161b) to head (5230198).
Report is 19 commits behind head on master.

Files with missing lines	Patch %	Lines
...rc/Swashbuckle.AspNetCore.ReDoc/ReDocMiddleware.cs	88.88%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3088      +/-   ##
==========================================
+ Coverage   90.50%   91.44%   +0.93%     
==========================================
  Files          76       76              
  Lines        3118     3121       +3     
  Branches      516      519       +3     
==========================================
+ Hits         2822     2854      +32     
+ Misses        296      267      -29     

Flag	Coverage Δ
Linux	91.44% <91.66%> (+0.93%)	⬆️
Windows	91.44% <91.66%> (+0.93%)	⬆️
macOS	91.44% <91.66%> (+0.93%)	⬆️

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[mesMess]

Conditional compilation to cover various NET runtimes
Consolidation of some partial classes
Support for ReDoc in AOT apps