[lib] Require primaryIdentityPublicKey in SIWEStatement

Summary:
We were previously accepting SIWEMessages without primaryIdentityPublicKey before we were able to integrate olm into web. Now that we've been able to integrate olm into web and are always including the primaryIdentityPublicKey in the SIWEMessage statement, we can validate that it's being included.

We don't have to worry about native clients that aren't including primaryIdentityPublicKey because none were publicly released.

Test Plan: Able to SIWE on both web and native without issue (against local keyserver).

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6826