Adding private_data to _BlockData

[emma58]

Fixes # .

Summary/Motivation:

We've maintained for awhile that we should not store state on transformation objects, and that we should instead do it on the model. That's all well and good except that we have never reserved a place for ourselves to do so. This adds a _private_data_dict attribute on _BlockData that is initialized as None, but when asked for (and given a scope (that is either the caller's module name or a parent of the caller's module name)), becomes a dictionary with keys being the name of the scope (e.g. pyomo.gdp or pyomo.gdp.bigm) and values being dictionaries to be used however the transformation/solver/anything else desires. This should simplify building and maintaining APIs for transformations to map between original and transformed components and store other state that might be needed later, and is actually private because of the restriction on keys. Even if a user used this, they would have to use keys from their scope, not ours.

Changes proposed in this PR:

• Adds _private_data property and private_data method to _BlockData
• Adds a test for the above.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f81f34f) 88.34% compared to head (170acb8) 88.33%.
Report is 38 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3138      +/-   ##
==========================================
- Coverage   88.34%   88.33%   -0.01%     
==========================================
  Files         833      833              
  Lines       92567    92579      +12     
==========================================
+ Hits        81776    81779       +3     
- Misses      10791    10800       +9     

Flag	Coverage Δ
linux	86.23% <100.00%> (+<0.01%)	⬆️
osx	75.69% <100.00%> (+<0.01%)	⬆️
other	86.42% <100.00%> (-0.01%)	⬇️
win	83.64% <100.00%> (-0.01%)	⬇️

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