Consistent annotation behavior without application_packages

## Describe the bug

When using the [public API](https://www.elastic.co/guide/en/apm/agent/java/current/public-api.html) annotations, the `application_packages` configuration is required to make them work.

This behavior is not consistent with other similar instrumentation and does not provide a good onboarding experience, as when configuration is missing nothing is instrumented.

For example, Web-services annotated with `@javax.jws.WebService` work out of the box without configuration, the effect of `application_packages` in this case is only to provide better performance as it allows to reduce the search scope for classes that might have the annotation (this search is by definition quite time and resources intensive).

Here, a better behavior would be:
- instrument classes by default, wether `application_packages` is set or not
- make sure that we issue a proper warning when `application_packages` is not set as it implies sub-optimal performance.
- make this behavior consistent on all plugins that relies on annotations.