[identity] verify nonce in siwe message

Summary:
get the nonce from the SIWE message provided by the user and check if it exists in our nonces table. if it doesn't, return an error. if it does, remove it and proceed.

Depends on D9440

Test Plan: tried to use the same nonce for consecutive sign-in requests and only the first request succeeded.

Reviewers: kamil, bartek, jon

Reviewed By: bartek

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9441