New endpoint for signing party- and account-level agreements

Broker API

Adds POST /v1/agreements to the Broker API reference, letting you sign agreements for an existing party or account after it has been created.

The request body is a CreateAgreementRequest accepting either a party-level agreement (CreatePartyAgreementRequest) or an account-level agreement (CreateAccountAgreementRequest).
Agreement types are enumerated by PartyAgreementType and PerAccountAgreementType, covering late additions such as certifications_and_resolutions_statement.
Successful responses return a SignedAgreement with a closed SignedAgreementType.
Errors are documented as 400 (party_not_found) and 403 (forbidden) using the PartyErrorResponse schema, along with restrictions on revising an already signed agreement.
Adds an Agreements tag to the reference.