[lib][native][web] Move callSingleKeyserverEndpoint to lib/keyserver-conn

Summary:
The utility was built before this folder was introduced. It seems like an obviously better location for it given the name and function.

Depends on D11696

Test Plan: Flow

Reviewers: tomek, inka

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D11697