feat(dsp): replace consumerId and providerId with odrl assignee and assigner

[ndr-brt]

What this PR changes/adds

Map assigner and assignee fields in the Policy object in the transformers.
Ensures that every Offer has the assigner property set.

Why it does that

DSP compliance

Further notes

• in the model objects like ContractAgreement I kept the current nomenclature (consumerId and providerId), to avoid disruptive changes

Linked Issue(s)

Closes #3792
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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.54839% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 73.09%. Comparing base (7f20ba5) to head (486b433).
Report is 100 commits behind head on main.

Files	Patch %	Lines
...onObjectToContractAgreementMessageTransformer.java	77.77%	0 Missing and 2 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3926      +/-   ##
==========================================
+ Coverage   71.74%   73.09%   +1.34%     
==========================================
  Files         919      934      +15     
  Lines       18457    18831     +374     
  Branches     1037     1072      +35     
==========================================
+ Hits        13242    13764     +522     
+ Misses       4756     4608     -148     
  Partials      459      459              

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