refactor: allow the scheduler to decide if an app is deployed

Without moving this to the scheduler, all applications are assumed to not be deployed unless the scheduler is set to docker-local.

Closes #3531

docs/development/plugin-triggers.md

@@ -1320,6 +1320,24 @@ DOKKU_SCHEDULER="$1"; APP="$2";
 # TODO
 ```
 
+### `scheduler-is-deployed`
+
+> Warning: The scheduler plugin trigger apis are under development and may change
+> between minor releases until the 1.0 release.
+
+- Description: Allows you to check if an app has been deployed
+- Invoked by: `dokku ps:rebuild`
+- Arguments: `$DOKKU_SCHEDULER $APP`
+- Example:
+
+```shell
+#!/usr/bin/env bash
+
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+DOKKU_SCHEDULER="$1"; APP="$2";
+
+# TODO
+```
 ### `scheduler-logs`
 
 > Warning: The scheduler plugin trigger apis are under development and may change