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>

scripts/drone/pipelines/build.star

@@ -4,6 +4,7 @@ load(
     "scripts/drone/steps/lib.star",
     "build_frontend_package_step",
     "build_storybook_step",
+    "build_test_plugins_step",
     "cloud_plugins_e2e_tests_step",
     "compile_build_cmd",
     "download_grabpl_step",
@@ -93,6 +94,7 @@ def build_e2e(trigger, ver_mode):
 
     build_steps.extend(
         [
+            build_test_plugins_step(),
             grafana_server_step(),
             e2e_tests_step("dashboards-suite"),
             e2e_tests_step("scenes/dashboards-suite"),