[services][identity] wallet login

Summary:
Full implementation of wallet login. Steps:

1. Parse SIWE message and signature from request
2. Verify message using siwe-rs crate
3. If verification is successful, generate a token, store it in the token table in DynamoDB, and then return it to the caller

Note that PAKE login is not covered here. It will be in a separate diff.

Test Plan: Successfully signed in with a metamask wallet

Reviewers: karol-bisztyga, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4285