A signer can use this operation to sign a signature request by delegating the signature to the platform.
When using this signature mode, the signature key of the signer is controled by the platform, which acts as a trusted third party.
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 delegate a signature
(authentication of the signer rely on the knowledge of his secret identifier and OTP).