[lib] Move createSIWEMessage from landing => lib
Summary: Just a simple drag/drop so it can be reused from web.
Test Plan:
Native SIWE flow continues to work as expected:
- Made a new wallet in Rainbow
- Tried native SIWE flow
- Was able to log in as expected
Reviewers: ashoat, tomek
Reviewed By: ashoat
Differential Revision: https://phab.comm.dev/D6306