[identity] CompareUsers RPC

Summary:
This RPC takes a list of users from the keyserver and returns any users in DynamoDB that are not in that list of users

One use case for this RPC is to ensure data completeness and correctness before we make Identity service the single source of truth

Adding ashoat since we're introducing a new RPC

Test Plan:
this will eventually be called from a cronjob, but I simulated this by adding a compare_users client method to the keyserver and
calling the RPC with:

1. an empty list -> got back the full list of DDB user IDs
2. exactly the same list of users as in my DDB test table -> got back an empty list
3. a list of random IDs not found in DDB -> got back the full list of DDB user IDs

Reviewers: jon, tomek, bartek, ashoat

Reviewed By: jon, bartek

Subscribers: atul

Differential Revision: https://phab.comm.dev/D6827