feat: implement the scheduler plugin

This also performs a one-time migration of the DOKKU_SCHEDULER values the scheduler plugin properties.

Closes #4739

plugins/scheduler/scheduler.go

@@ -0,0 +1,13 @@
+package scheduler
+
+var (
+	// DefaultProperties is a map of all valid network properties with corresponding default property values
+	DefaultProperties = map[string]string{
+		"selected": "docker-local",
+	}
+
+	// GlobalProperties is a map of all valid global network properties
+	GlobalProperties = map[string]bool{
+		"selected": true,
+	}
+)