Extract shared logci for SIWE panel state handling

Summary: The way ConnectEthereum and CreateSIWEBackupMessage handle SIWE panel state is exactly the same. Additionally getting signature for existing backup message will resuse this logic as well. Therefore this diff extracts it.

Test Plan: Test that SIWE registration, log in and backup message creation are unchanged.

Reviewers: kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12032