[services] consolidate ddb client code, small refactor

Summary: consolidated some of the client code to minimize the number of calls we make to DynamoDB. also changed the PAKE_USER_ prefix to USER in constants.rs since the table holds more than just PAKE data now.

Test Plan: cargo build, tested that the update expression worked by creating, updating an item in the DDB table

Reviewers: tomek, karol

Reviewed By: tomek, karol

Subscribers: ashoat, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4626