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.

• Request body accepts either a party-level agreement (CreatePartyAgreementRequest) or an account-level agreement (CreateAccountAgreementRequest), with closed enums for the supported agreement types.
• Returns a SignedAgreement describing the recorded signature, including its SignedAgreementType.
• Documents the error responses: 400 for an unknown party and 403 when signing is not permitted.
• Documents the restrictions on signing a revised agreement.
• Adds an Agreements tag to group the operation.

Affected Endpoints

• POST /v1/agreements