[web] implementing getting inbound user keys in IdentityServiceClient

Summary:
Call RPC to get the keys of all devices of a given user. In case of inbound session we know the device(s)ID so it's more convenient to return object than array.

Depends on D11106

Test Plan: Call method and log result

Reviewers: marcin, varun, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11107