[lib][native][web] Parse keyserver keys payload and validate the result

Summary:
A signature of the returned keys changes - the new one is more convenient to use with Olm. Additionally, this diff introduces more efficient validation.

Depends on D10665

Test Plan: Check if fetching keys returns correct values on native and web.

Reviewers: varun, inka, kamil

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10666