[lib] Introduce identityKeysBlobValidator and use in login/register/siwe_auth responders

Summary: identityKeysBlobValidator ensures that the structure of the JSON.parse()d signedIdentityKeysBlob.payload is correct and throws a ServerError if it isn't.

Test Plan: Made sure login/register/siwe_auth works on native. Made sure login/siwe_auth works on web.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6950