[services][identity] create a struct for the users table in MySQL

Summary:
this struct will store the data we need to move from the users table in MySQL to DynamoDB. we are omitting the password hash since we're not storing passwords on the Identity service, avatar since it's unused currently, and public_key since keys will be per-device.

Adding @ashoat on these DB-related diffs since he has a lot of context

Test Plan: cargo build (this is dead code so it can't break anything)

Reviewers: atul, ashoat, jon

Reviewed By: atul, ashoat

Subscribers: derek, tomek, abosh, ashoat

Differential Revision: https://phab.comm.dev/D5448