Plugin extensions: Registry should reject Grafana core extensions that targets plugins

**Why is this needed**: As specified in the [best practices](https://grafana.com/developers/plugin-tools/key-concepts/best-practices#ui-extensions) for plugin development, Grafana core should not extend plugins. **What would you like to be added**: I would like the plugin extension registry to reject extensions defined in Grafana core that targets extensions points defined in plugins. **Who is this feature for?** Everyone