feat: allow specifying app level feature flags

application feature flags can be defined in the application
creation. see docs/plugins/feature-flags.md for reference.

closes #15553

Signed-off-by: Heikki Hellgren <heikki.hellgren@op.fi>

packages/app-defaults/src/createApp.tsx

@@ -51,6 +51,7 @@ export function createApp(
       ...options?.icons,
     },
     plugins: (options?.plugins as BackstagePlugin[]) ?? [],
+    featureFlags: options?.featureFlags ?? [],
     themes: options?.themes ?? themes,
   });
 }