Change SIWE panel props to enable passing complete SIWE message

Summary: This differential changes SIWE panel props so that it is possible to pass complete message to sign.

Test Plan: Check that SIWE features are not broken.

Reviewers: kamil, tomek, ginsu, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12034