Introduce UI PluginsService (#32672) (#34498)

This commit introduces two changes:
- Adds new platform plugins as a new bundles to the optimizer
- A PluginsService in the UI that loads plugin bundles, initializes plugins, and manages the lifecycle of plugins.

test/plugin_functional/config.js

@@ -33,6 +33,7 @@ export default async function ({ readConfigFile }) {
       require.resolve('./test_suites/custom_visualizations'),
       require.resolve('./test_suites/embedding_visualizations'),
       require.resolve('./test_suites/panel_actions'),
+      require.resolve('./test_suites/core_plugins'),
     ],
     services: functionalConfig.get('services'),
     pageObjects: functionalConfig.get('pageObjects'),
@@ -51,6 +52,8 @@ export default async function ({ readConfigFile }) {
       serverArgs: [
         ...functionalConfig.get('kbnTestServer.serverArgs'),
         ...plugins.map(pluginDir => `--plugin-path=${path.resolve(__dirname, 'plugins', pluginDir)}`),
+        // Required to load new platform plugins via `--plugin-path` flag.
+        '--env.name=development',
       ],
     },
   };