[services][identity] add new client method to add users to user table

Summary:
we should call the dynamodb put_item API instead of update_item to add new items to the table.

this change introduces a new db client method that calls the put_item API.

Test Plan: called the db client method from main and successfully added a new user to a test table in ddb

Reviewers: jon, tomek, bartek

Reviewed By: bartek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6691