Introduce new header to siwe endpoint

Summary: Signing already created backup message is new usage of siwe endpoints. Additionally it is the first time siwe endpoint is not supposed to generate message itself. This differential add additional header to the endpoint that can carry complete backup message. Invariant protects against sending complete message along with nonce in headers. Finally this diff introduces new copy that will be displayed to the user when signing backup message.

Test Plan: For now keep testing that all SIWe features (log in, backup message and register) are not broken

Reviewers: ashoat, kamil, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D12033