Refactor: replace policy with policyId on ContractAgreement

[ndr-brt]

What this PR changes/adds

Replace Policy instance with policyIs on ContractAgreement

Why it does that

To avoid data duplication

Further notes

• It's the second point of Refactor structure of business objects #828 and the final one
• PolicyArchive now relies on PolicyStore to get the Policy instance

Linked Issue(s)

Closes #828

Checklist

• added appropriate tests?
• performed checkstyle check locally?
• added/updated copyright headers?
• documented public classes/methods?
• added/updated relevant documentation?
• added relevant details to the changelog? (skip with label no-changelog)
• formatted title correctly? (take a look at the CONTRIBUTING and styleguide for details)

[codecov-commenter]

Codecov Report

Merging #1220 (219aecb) into main (2e21e5a) will decrease coverage by 0.01%.
The diff coverage is 60.86%.

@@             Coverage Diff              @@
##               main    #1220      +/-   ##
============================================
- Coverage     58.63%   58.62%   -0.02%     
- Complexity     2731     2736       +5     
============================================
  Files           698      700       +2     
  Lines         15451    15502      +51     
  Branches       1049     1032      -17     
============================================
+ Hits           9060     9088      +28     
- Misses         5967     5997      +30     
+ Partials        424      417       -7     

Impacted Files	Coverage Δ
...tcher/sender/MultipartContractAgreementSender.java	0.00% <0.00%> (ø)
...pi/multipart/handler/ContractAgreementHandler.java	0.00% <0.00%> (ø)
.../transform/ContractAgreementTransformerOutput.java	0.00% <0.00%> (ø)
...or/ids/transform/IdsTransformServiceExtension.java	80.00% <ø> (-5.72%)	⬇️
.../contractnegotiation/store/PostgresStatements.java	3.12% <0.00%> (-0.11%)	⬇️
...negotiation/store/SqlContractNegotiationStore.java	4.44% <0.00%> (+0.04%)	⬆️
...s/domain/contract/agreement/ContractAgreement.java	0.00% <0.00%> (ø)
...n/contract/agreement/ContractAgreementRequest.java	0.00% <0.00%> (ø)
...ementRequestToIdsContractAgreementTransformer.java	75.51% <75.51%> (ø)
...ceconnector/contract/ContractServiceExtension.java	95.08% <100.00%> (+0.16%)	⬆️
... and 33 more

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