[identity] return user identifier in InboundKeysForUserResponse

Summary: Receivers will need the outbound user's username/wallet address. They should get it from the identity service, rather than rely on the outbound user to provide the correct value.

Test Plan:
tested this on staging by calling the get_inbound client method from my local keyserver and successfully retrieving a username for an existing user.
tried again with a wallet user and got back a wallet address.

Reviewers: bartek, ashoat, will

Reviewed By: bartek, ashoat

Subscribers: inka, kamil, will, ashoat, tomek

Differential Revision: https://phab.comm.dev/D10560