New options and mappings in the core.add_slack_variables Transformation

[emma58]

Fixes # .

Summary/Motivation:

We've always used the core.add_slack_variables transformation as purely a debugging tool for infeasible models. However, there are other reasons one could want to add slack variables, such as relaxing certain constraints of a model by bounded amounts. This PR adds a couple features to the add slack variables transformation to make that easier, specifically an API so that a user can get the slack variables from the constraints and adding an option to not change the objective.

Changes proposed in this PR:

• Adds a config option to not touch the objective
• Adds public methods for getting relaxed constraints corresponding to given slack variables and vice versa on the transformation

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.60%. Comparing base (7ad143c) to head (155815b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3869      +/-   ##
==========================================
- Coverage   89.67%   89.60%   -0.07%     
==========================================
  Files         908      908              
  Lines      106735   106761      +26     
==========================================
- Hits        95717    95666      -51     
- Misses      11018    11095      +77     

Flag	Coverage Δ
builders	29.08% <38.70%> (+<0.01%)	⬆️
default	85.96% <100.00%> (?)
expensive	35.51% <38.70%> (?)
linux	86.71% <100.00%> (-2.46%)	⬇️
linux_other	86.71% <100.00%> (-0.43%)	⬇️
oldsolvers	28.00% <38.70%> (+<0.01%)	⬆️
osx	82.04% <100.00%> (+<0.01%)	⬆️
win	85.14% <100.00%> (-0.42%)	⬇️
win_other	85.14% <100.00%> (-0.42%)	⬇️

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