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/steps/lib.star

@@ -470,6 +470,26 @@ def build_frontend_step():
         ],
     }
 
+def build_test_plugins_step():
+    """Build the test plugins used in e2e tests
+
+    Returns:
+      Drone step.
+    """
+    return {
+        "name": "build-test-plugins",
+        "image": images["node"],
+        "environment": {
+            "NODE_OPTIONS": "--max_old_space_size=8192",
+        },
+        "depends_on": [
+            "yarn-install",
+        ],
+        "commands": [
+            "yarn e2e:plugin:build",
+        ],
+    }
+
 def update_package_json_version():
     """Updates the packages/ to use a version that has the build ID in it: 10.0.0pre -> 10.0.0-5432pre
 
@@ -773,6 +793,7 @@ def e2e_tests_step(suite, port = 3001, tries = None):
         "image": images["cypress"],
         "depends_on": [
             "grafana-server",
+            "build-test-plugins",
         ],
         "environment": {
             "HOST": "grafana-server",
@@ -872,6 +893,7 @@ def playwright_e2e_tests_step():
         "image": images["node_deb"],
         "depends_on": [
             "grafana-server",
+            "build-test-plugins",
         ],
         "commands": [
             "npx wait-on@7.0.1 http://$HOST:$PORT",