[lib] Introduce getSIWEStatementForPublicKey(...)
Summary:
Given a public key, getSIWEStatementForPublicKey(...) generates a statement for SIWE message that conforms to what is described in the whitepaper:
Depends on D6128
Test Plan: included a unit test
Reviewers: ashoat, tomek
Reviewed By: ashoat
Differential Revision: https://phab.comm.dev/D6129