test: Add tests to monitor any changes in the public API surface in NewRelic.Api.Agent

[tippmar-nr]

Adds a unit test project that leverages the PublicAPIGenerator and Verify NuGet packages as suggested in Andrew Lock's blog.

The test will monitor the public API in NewRelic.Api.Agent and will fail if there is a change. When running the test locally, a diff tool will be launched (using VS Code in most cases) that will help to resolve the failure by showing the change:

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.17%. Comparing base (32e54c1) to head (3fe7a33).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2749   +/-   ##
=======================================
  Coverage   81.17%   81.17%           
=======================================
  Files         459      459           
  Lines       29204    29204           
  Branches     3223     3223           
=======================================
  Hits        23705    23705           
  Misses       4708     4708           
  Partials      791      791           

Flag	Coverage Δ
Agent	82.18% <ø> (ø)
Profiler	72.12% <ø> (ø)

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