Report feedback about a signature request.

A signer can use this operation to report a feedback to the owner of a signature request.

This operation is only available when the email of the signer is set: since the secret identifier is sent by email to the signer, he/she can provide it back to authenticate.

This is a publicly accessible endpoint: authentication is not required to report feedback
(authentication of the signers, when required, rely on their knowledge of their secret identifier).