Add daemon overseer modules to allow daemons to be externally reloaded

Summary: Ref T7053. Add a PhutilDaemonOverseerModule class which can be used to externally influence daemons.

Test Plan: See D14458.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T7053

Differential Revision: https://secure.phabricator.com/D14452