Add contact information field to counterparties in POST endpoints

Summary

This change adds a new optional contact field to counterparty data when creating or updating records via the API, allowing partners to store contact information for counterparties.

Updated endpoints

• POST /counterparties
  • Added an optional contact property to the counterparty creation payload for providing contact information.
• POST /loans
  • Updated to include the contact property inside the embedded counterparty data.

Other Changes

• Extended the APICounterpartyExplicitCreation, APICounterpartyWithExposure, and CounterpartyCreation schema definitions to include a new nullable contact field, referencing the APICounterpartyContact schema.