Support a reusable 'var_cache' within the IdentifyVariableVisitor

[jsiirola]

Fixes # .

Summary/Motivation:

This implements a minor performance improvement when we want to collect all the variables used by a model. The previous implementation ended up using two "seen" caches: one within the IdentifyVariableVisitor, and the other in the get_var_from_components() utility. This allows a client to instantiate an IdentifyVariableVisitor with a persistent "seen" cache, thereby eliminating the redundant caches.

This implementation is fully backwards compatible with the previous IdentifyVariableViaitor / identify_variables implementation.

Changes proposed in this PR:

• Support a persistent "seen" cache within the IdentifyVariableVisitor
• Update get_vars_from_components to use a single persistent "seen" cache
• Add a get_vars() utility that is an alias for get_vars_from_components(ctype=(Constraint, Objective))
• Test the new behavior

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.12%. Comparing base (3603b1e) to head (33064ab).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3966   +/-   ##
=======================================
  Coverage   90.11%   90.12%           
=======================================
  Files         909      909           
  Lines      108580   108641   +61     
=======================================
+ Hits        97847    97912   +65     
+ Misses      10733    10729    -4     

Flag	Coverage Δ
builders	29.10% <23.80%> (+<0.01%)	⬆️
default	86.11% <100.00%> (?)
expensive	35.13% <95.23%> (?)
linux	87.63% <100.00%> (-1.99%)	⬇️
linux_other	87.63% <100.00%> (+<0.01%)	⬆️
oldsolvers	28.04% <23.80%> (+<0.01%)	⬆️
osx	83.05% <100.00%> (+<0.01%)	⬆️
win	85.37% <100.00%> (+<0.01%)	⬆️

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