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).

docs/usage/configuration/config_documentation.md

@@ -230,6 +230,13 @@ Example configuration:
 presence:
   enabled: false
 ```
+
+`enabled` can also be set to a special value of "untracked" which ignores updates
+received via clients and federation, while still accepting updates from the
+[module API](../../modules/index.md).
+
+*The "untracked" option was added in Synapse 1.96.0.*
+
 ---
 ### `require_auth_for_profile_requests`