Add overloads to AtUrl and AtAbsoluteUrl which can use a IStringMatcher

[StefH]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 35.65%. Comparing base (deda7fb) to head (609f237).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/WireMock.Net/Matchers/MatcherExtensions.cs	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1221      +/-   ##
==========================================
+ Coverage   33.89%   35.65%   +1.76%     
==========================================
  Files         201      228      +27     
  Lines       33154    41375    +8221     
==========================================
+ Hits        11236    14753    +3517     
- Misses      21407    26037    +4630     
- Partials      511      585      +74     

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