Sign a signature request by delegating the signature.

A signer can use this operation to sign a signature request by delegating the signature to the platform.

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.

When using this signature mode, the signer's identity and signature key is controled by the platform, which act 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 signers, when required, rely on their knowledge of their secret identifier and OTP, or on their control of their public key).