feat: specify global metadata with -m|--metadata arg

[yufeih]

Support specifying global metadata through the -m or --metadata command line argument, e.g., docfx build --metadata _appTitle="My App 1":

• Set multiple metadata by passing multiple --metadata args
• true and false are parsed as boolean
• The value of true can be omitted, e.g, docfx build --metadata _disableContribution

fixes #8787

[codecov]

Codecov Report

Patch coverage: 22.22% and project coverage change: -0.03 ⚠️

Comparison is base (3e73134) 76.84% compared to head (ab7a529) 76.82%.

❗ Current head ab7a529 differs from pull request most recent head b4964d9. Consider uploading reports for the commit b4964d9 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8816      +/-   ##
==========================================
- Coverage   76.84%   76.82%   -0.03%     
==========================================
  Files         606      606              
  Lines       25041    25050       +9     
==========================================
+ Hits        19243    19244       +1     
- Misses       5798     5806       +8     

Impacted Files	Coverage Δ
src/docfx/Models/BuildCommand.cs	40.38% <12.50%> (-5.07%)	⬇️
src/docfx/Models/BuildCommandOptions.cs	90.47% <100.00%> (+0.47%)	⬆️

... and 1 file with indirect coverage changes

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