Sign a custody contract on behalf of the session's group

Records the user's signature on the contract template identified by contract_type. An unknown contract_type is 400 invalid_contract_type. A click-through that e-signature decides is refused with 409 esign_required: while an e-signature request for that contract is in progress or has been signed, or while the user signs their business's custody contracts by e-signature (custody_contracts_enrolled on GET /compliance/esign/enrollment). The user then signs, or has signed, through the e-signature request instead.