Sign a signature request by registering a signature.

A signer can use this operation to sign a signature request by registering a signature he/she procuded on his/her own.

The signature is automatically anchored on behalf of the owner of the signature request.

The signature anchor created is added to the list of signature anchors of the signature request.

This is a publicly accessible endpoint: authentication is not required to register a signature
(authentication of the signers, when required, rely on their knowledge of their secret identifier and OTP, or on their control of their public key).