[web] implementing getting outbound user keys in IdentityServiceClient

Summary:
Call RPC to get the keys of all devices of a given user.

This approach is different, we want to:

1. Return all user devices
2. Return valid user keys even though one device has for example missing one-time keys
   
   Depends on D10748

Test Plan: Call method and log result

Reviewers: tomek, varun

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10749