Update PauseGC to work correctly in nested contexts

[jsiirola]

Fixes # .

Summary/Motivation:

This PR updates the PauseGC context manager so that it works correctly in nested contexts.

Changes proposed in this PR:

• Update PauseGC to restore the original state of the garbage collector on context manager exit
• Add a "pseudo stack" for tracking nested instances of the PauseGC context manager
• Update tests to verify nested operation

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
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[codecov]

Codecov Report

Merging #2507 (1a0ce99) into main (fb050d2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2507   +/-   ##
=======================================
  Coverage   86.00%   86.01%           
=======================================
  Files         715      715           
  Lines       80117    80130   +13     
=======================================
+ Hits        68908    68921   +13     
  Misses      11209    11209           

Flag	Coverage Δ
linux	83.44% <100.00%> (+<0.01%)	⬆️
osx	73.76% <100.00%> (+<0.01%)	⬆️
other	83.62% <100.00%> (+<0.01%)	⬆️
win	80.60% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
pyomo/common/gc_manager.py	100.00% <100.00%> (ø)

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