[native] keyserverUsername -> keyserverURL

Summary:
We're going to use a URL for the keyserver for now, since the identity service isn't ready for querying keyservers by username yet.

This is a simple rename diff.

Depends on D8192

Test Plan: Flow

Reviewers: atul, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8193