[native] Expose get keyserver keys Identity RPC

Summary:
Add Rust and C++ code that allows calling this function.

https://linear.app/comm/issue/ENG-6083/allow-calling-getkeyserverkeys-on-native

Depends on D10403

Test Plan: Call this function in a code introduced by D10327 and check if the keys are returned.

Reviewers: varun, bartek, michal, kamil

Reviewed By: bartek

Subscribers: kamil, ashoat

Differential Revision: https://phab.comm.dev/D10449