Introduce SIWE message types

Summary:
This differential introduces the type for message that we want to sign with SIWE and passes it as a header to SIWE WebView. Future differentials will render different
user texts based on the value in this header.

Test Plan:

1. Flow
2. Test that siwe works on native and web as it used to.

Reviewers: atul, inka, tomek

Reviewed By: atul

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11633