[Graph] disable application in serverless (#163582)

## Summary

Closes https://github.com/elastic/kibana/issues/163340

This PR disables graph in serverless. Specifically:

- creates a serverless yml setting for disabling graph
- adds the setting in serverless.yml

config/serverless.yml

@@ -90,6 +90,7 @@ vis_type_timeseries.readOnly: true
 vis_type_vislib.readOnly: true
 vis_type_xy.readOnly: true
 input_control_vis.readOnly: true
+xpack.graph.enabled: false
 
 # Disable cases in stack management
 xpack.cases.stack.enabled: false

x-pack/plugins/graph/config.ts

@@ -18,6 +18,12 @@ export const configSchema = schema.object({
     { defaultValue: 'configAndData' }
   ),
   canEditDrillDownUrls: schema.boolean({ defaultValue: true }),
+  enabled: schema.conditional(
+    schema.contextRef('serverless'),
+    true,
+    schema.maybe(schema.boolean({ defaultValue: true })),
+    schema.never()
+  ),
 });
 
 export type ConfigSchema = TypeOf<typeof configSchema>;