feat(dsp): implement GET /id transfer process endpoint

[ronjaquensel]

What this PR changes/adds

Adds a method findById to the TransferProcessProtocolService, that takes a ClaimToken as parameter and returns a transfer process only if the requesting party is authorized. Using this method, implements the GET /id endpoint in the DspTransferProcessApiController.

Why it does that

To support the protocol specification

Further notes

Adds a second validateRequest() method to the ContractValidationService, that checks whether a requesting participant is authorized for a contract agreement, but does not perform policy evaluation.

Linked Issue(s)

Closes #2761

[codecov-commenter]

Codecov Report

Patch coverage: 88.46% and project coverage change: +0.01 🎉

Comparison is base (070aa11) 65.69% compared to head (c444022) 65.71%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3234      +/-   ##
==========================================
+ Coverage   65.69%   65.71%   +0.01%     
==========================================
  Files         840      840              
  Lines       16787    16805      +18     
  Branches      917      918       +1     
==========================================
+ Hits        11028    11043      +15     
- Misses       5392     5394       +2     
- Partials      367      368       +1     

Impacted Files	Coverage Δ
...pi/controller/DspTransferProcessApiController.java	92.50% <76.92%> (-2.17%)	⬇️
...ract/validation/ContractValidationServiceImpl.java	86.00% <100.00%> (+0.73%)	⬆️
...ferprocess/TransferProcessProtocolServiceImpl.java	98.87% <100.00%> (+0.11%)	⬆️

... and 1 file with indirect coverage changes

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