[services] implement getUserID rpc

Summary:
This RPC lets a client get a user ID corresponding to a given wallet address or username.

Depends on D4657

Test Plan: Ran the service and called the RPC, retrieved the correct user ID from DynamoDB with a wallet address and a username

Reviewers: tomek, karol

Reviewed By: tomek

Subscribers: ashoat, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4658