Java SDK: native Dag definition

### Body **We should wait until [AIP-85 Dag Importers](https://cwiki.apache.org/confluence/spaces/AIRFLOW/pages/315494137/AIP-85+DAG+importer) pass**. ### Background The Java SDK (`1.0.0-beta1`) intentionally ships in "Python-stub-Dag mode": Java SDK handlers are registered for Dag/task IDs declared in Python. Higher-level Java Dag authoring was deferred until coordinator-mode execution is reviewed. ### What needs to happen 1. Allow declaring Dags natively in Java-SDK. 2. Revisit bundle ergonomics alongside the authoring interface. ### Acceptance criteria - A Dag can be authored end to end in Java with no Python stub file. ### Committer - [x] I acknowledge that I am a maintainer/committer of the Apache Airflow project.