Add a new module API to update user presence state. (#16544)

This adds a module API which allows a module to update a user's
presence state/status message. This is useful for controlling presence
from an external system.

To fully control presence from the module the presence.enabled config
parameter gains a new state of "untracked" which disables internal tracking
of presence changes via user actions, etc. Only updates from the module will
be persisted and sent down sync properly).