[services][identity] add cargo run subcommand for populating DDB from MySQL

Summary:
We want to make sure that all the relevant existing user data is moved over before we begin double-writing user data to DynamoDB.
The script to handle moving this data will be a subcommand of the Identity service, because that lets us re-use some of the DynamoDB client code.

Test Plan: cargo run in the identity service directory displays the new subcommand

Reviewers: tomek, atul, jon

Reviewed By: tomek

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D5392