Create an Agreement against a KYC-trusted account holder's bank account.

The calling account must be flagged as kyc_trusted. If it is not, a
400 Bad Request will be returned with must_be_kyc_trusted.

When the optional bank connection attributes (basiq_user_id,
basiq_connection_id, proviso_reference, credit_sense_app_id) are
provided, the Agreement is created in the unverified state and bank
connection verification is queued asynchronously. When verification
succeeds the Agreement transitions to accepted; when it fails the
Agreement transitions to declined and the failure reason is recorded
on status_reason. The verification outcome is not reflected in this
endpoint's response - listen for the corresponding agreement.accepted
or agreement.declined webhook to observe the final state.

When no bank connection attributes are provided, the Agreement is
auto-accepted before the response is returned.