[native] implement getOutboundKeys method in CommRustModule

Summary:
we need to get the keyserver's keys to set up an Olm session on the client.

Depends on D9345

Test Plan: successfully retrieved my local keyserver's keys by calling the CommRustModule method from js

Reviewers: kamil, bartek

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9346