Add command line option to specify URL schema override for XmlSchemaSet

[phoyd]

See #534. Makes XmlSchemaSet resolve URLs that differ only by schema prefix (http: and https: for example) as the same URL.

[codecov]

Codecov Report

Attention: Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 94.10%. Comparing base (e44cd71) to head (1afc95e).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
XmlSchemaClassGenerator/Generator.cs	95.65%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #535      +/-   ##
==========================================
+ Coverage   94.08%   94.10%   +0.01%     
==========================================
  Files          21       21              
  Lines        3164     3187      +23     
  Branches      496      497       +1     
==========================================
+ Hits         2977     2999      +22     
- Misses        122      123       +1     
  Partials       65       65              

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[mganss]

Thanks! Could you add a quick unit test?