[keyserver] Send macos notifications

Summary: Actually send the notification to all macos devices. One things that is different is that we don't include text for alert notif so it doesn't display automatically by macos but we handle the visible notification ourselves. More information in ENG-3056.

Test Plan: Tested with the next diff, if the notifications are sent correctly.

Reviewers: tomek, inka, kamil, ashoat

Reviewed By: ashoat

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6893