[lib][keyserver] Allow notificationTexts to render EntityText

Summary:
This diff adds support to keyserver for resolving EntityText returned by a message spec's notificationTexts function.

Depends on D6573

Test Plan: Tested in combination with later notif diffs

Reviewers: atul, ginsu, michal, marcin

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6574