[web] Expose get keyserver keys Identity RPC

Summary:
Add a function and map its result to a JS object.

https://linear.app/comm/issue/ENG-6082/allow-calling-getkeyserverkeys-on-web

Test Plan: Call this function with hardcoded credentials taken from native and check if the keys are returned.

Reviewers: inka, varun, bartek

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10403