[lib] Introduce getPublicKeyFromSIWEStatement(...)

Summary:
Introduce getPublicKeyFromSIWEStatement(...) to pull public key out of SIWE statement which includes a public key. Will be used on the keyserver to retrieve the public key from the signed SIWE message.

Depends on D6130

Test Plan: included a unit test (will include more later)

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6131