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/README.md

@@ -0,0 +1,33 @@
+# Test plugins
+
+The [e2e test server](https://github.com/grafana/grafana/blob/main/scripts/grafana-server/start-server) automatically scans and looks for plugins in this directory.
+
+### To add a new test plugin:
+
+1. If provisioning is required you may update the YAML config file in [`/devenv`](https://github.com/grafana/grafana/tree/main/devenv).
+2. Add the plugin ID to the `allow_loading_unsigned_plugins` setting in the test server's [configuration file](https://github.com/grafana/grafana/blob/main/scripts/grafana-server/custom.ini).
+
+### Building a test plugin with webpack
+
+If you wish to build a test plugin with webpack, you may take a look at how the [grafana-extensionstest-app](./grafana-extensionstest-app/) is wired. A few things to keep in mind:
+
+- the package name needs to be prefixed with `@test-plugins/`
+- extend the webpack config from [`@grafana/plugin-configs`](../../packages/grafana-plugin-configs/) and use custom webpack config to only copy the necessary files (see example [here](./grafana-extensionstest-app/webpack.config.ts))
+- keep dependency versions in sync with what's in core
+
+#### Local development
+
+1: Install frontend dependencies:
+`yarn install --immutable`
+
+2: Build and watch the core frontend
+`yarn start`
+
+3: Build and watch the test plugins
+`yarn e2e:plugin:build:dev`
+
+4: Build the backend
+`make build-go`
+
+5: Start the Grafana e2e test server with the provisioned test plugin
+`PORT=3000 ./scripts/grafana-server/start-server`