Add global internal param to get default datapath worker pool

[ThadHouse]

Description

Before #4492, it was possible for apps statically linking to MsQuic and CxPLat to get the default worker pool used by Quic. After that PR, the default worker pool is no longer accessible. Add a new global param getter to make that accessable.

Testing

Added test to cover the new parameter.

Documentation

Internal, so not documented.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.93%. Comparing base (9f2f0d4) to head (1233899).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/core/library.c	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4535      +/-   ##
==========================================
- Coverage   86.19%   85.93%   -0.27%     
==========================================
  Files          56       56              
  Lines       15520    15530      +10     
==========================================
- Hits        13378    13346      -32     
- Misses       2142     2184      +42     

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