[keyserver] Check primaryIdentityPublicKeys.ed25519 against SIWE statement primaryIdentityPublicKey

Summary:
We want to make sure that the public key included in the SIWE statement matches that provided in the siwe_auth request.

Depends on D6909

Test Plan:
SIWE on web and set breakpoints and things were as expected.

Going to test native before landing to make sure siwe_auth continues working as expected.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6911