Add app maintenance spec with enabled field (#930)

The enabled field allows users to enable/disable maintenance mode.

Co-authored-by: Andrew Starr-Bochicchio <[email protected]>

specification/resources/apps/models/app_maintenance_spec.yml

@@ -0,0 +1,7 @@
+type: object
+description: Specification to configure maintenance settings for the app, such as maintenance mode.
+properties:
+  enabled:
+    type: boolean
+    description: Indicates whether maintenance mode should be enabled for the app.
+    example: true

specification/resources/apps/models/app_spec.yml

@@ -77,5 +77,8 @@ properties:
   egress:
     $ref: app_egress_spec.yml
 
+  maintenance:
+    $ref: app_maintenance_spec.yml
+
 required:
 - name