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

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

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