[identity] getInboundKeysForUser RPC implementation

Summary:
use previously implemented helpers to implement the actual RPC method

Note that this intentionally does not include one-time keys (not required for inbound, only outbound).

Depends on D8765

Test Plan: called the RPC for an existing user in my local DDB table and got back required keys.

Reviewers: jon, bartek

Reviewed By: jon, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8863