[backup-service] implement calling FindUserID RPC from Backup

Summary:
ENG-6145.

This is similar to other places in the codebase (here or here), there is also alternative where this code could be places in comm-lib but I prefer this, we can move it if needed.

Using is_valid_ethereum_address should be safe, we do the same on the client, and also it's not possible to create username which id valid ETH address.

Depends on D13805

Test Plan: tested later in the stack

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13806