[CMAKE] Add CMake script to find or fetch Microsoft.GSL

[dbarker]

Fixes # (issue)

Changes

• Add CMake script to find or fetch Microsoft.GSL
• Add Microsoft.GSL to the list of supported third party packages
• Test WITH_GSL through the Windows cmake.install.test

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[netlify]

✅ Deploy Preview for opentelemetry-cpp-api-docs canceled.

Name	Link
🔨 Latest commit	79aab53
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.96%. Comparing base (6c4d45c) to head (79aab53).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3521   +/-   ##
=======================================
  Coverage   89.96%   89.96%           
=======================================
  Files         219      219           
  Lines        7051     7051           
=======================================
  Hits         6343     6343           
  Misses        708      708           

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[owent]

LGTM, thanks.