Plugin Extensions: Only load app plugins when necessary (#86624)

* feat(plugins): automatically preload plugins

This PR enables auto-preloading for plugins when they are used
by an extension or extension-point. Once this change is merged plugins
that were only using "preload: true" in their plugin.json for using extensions
can remove it.

* fix: remove unused types

* fix: call `setComponentsFromLegacyExports()` after meta is initialised

e2e/test-plugins/grafana-extensionstest-app/README.md

@@ -32,4 +32,4 @@ Note that this plugin extends the `@grafana/plugin-configs` configs which is why
 
 ## Run Playwright tests
 
-- `yarn playwright --project extensions-test-app`
+- `yarn playwright test --project extensions-test-app`