[lib][keyserver] Convert notificationTexts to async functions

Summary: This will allow us to handle ENS resolution there. We'll probably actually do that outside of notificationTexts (probably in fullNotifTextsForMessageInfo), but it's probably good to make notificationTexts async as well (for eg. @ginsu's planned work on reactions notifs).

Test Plan: Flow

Reviewers: ginsu, atul, michal, marcin

Reviewed By: atul

Subscribers: tomek, ginsu

Differential Revision: https://phab.comm.dev/D6571