E2E: Add support for building test plugins (#91873)

* build test apps with webpack

* add extensions test app

* update e2e tests

* remove non-build test apps using amd

* use @grafana/plugin-configs rather than create-plugin config

* Update e2e/plugin-e2e/plugin-e2e-api-tests/as-admin-user/extensions/usePluginComponents.spec.ts

Co-authored-by: Jack Westbrook <jack.westbrook@gmail.com>

* Update package.json

Co-authored-by: Jack Westbrook <jack.westbrook@gmail.com>

* use run dir variable instead of hardcoded path

* add dummy licence file

* add separate step for building test plugins

* support nested plugins

* remove react-router-dom from the externals array

* remove add_mode dev

* lint starlark

* pass license path as env variable

* fix the path

* chore(e2e-plugins): clean up dependencies to match core versions

* refactor(e2e-plugins): prefer extending webpack plugins-config

* docs(e2e-plugins): add basic info to extensions test plugin readme

* update readme

* change dir name from custom plugins to test plugins

* change root readme

* update lockfile

---------

Co-authored-by: Jack Westbrook <jack.westbrook@gmail.com>

e2e/test-plugins/frontend-sandbox-datasource-test/plugin.json

@@ -0,0 +1,26 @@
+{
+  "$schema": "https://raw.githubusercontent.com/grafana/grafana/master/docs/sources/developers/plugins/plugin.schema.json",
+  "type": "datasource",
+  "name": "Sandbox datasource test plugin",
+  "id": "sandbox-test-datasource",
+  "metrics": true,
+  "info": {
+    "keywords": ["explore", "datasource"],
+    "description": "",
+    "author": {
+      "name": "Grafana"
+    },
+    "logos": {
+      "small": "img/logo.svg",
+      "large": "img/logo.svg"
+    },
+    "links": [],
+    "screenshots": [],
+    "version": "1.0.0",
+    "updated": "2023-06-27"
+  },
+  "dependencies": {
+    "grafanaDependency": ">=10.0",
+    "plugins": []
+  }
+}