[identity] Move X3DH RPCs to auth proto

Summary:
Copied the GetOutboundKeysForUser and GetInboundKeysForUser RPCs from the client proto to the auth proto.
Kept references to the original request and response messages because they're incompatible with existing ones in the auth proto.

This one is going to be further refactored as a part of ENG-5659.
Currently, left client side unchanged until that task is done.

Depends on D9931

Test Plan: Copy-paste so ensured Identity is building. The RPC and clients will be tested when refactored to use user ID instead of username/wallet address.

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9957