Gradle Extension for Java SDK

[jason810496]

Since we "hack" some build-time details, for example:

1. Pack dagIds, taskIds as an airflow-metadata.yaml in jar
2. Pack raw user's java file in the jar for BaseCoordinator.write_dag_code to get the raw code

Which means the normal ./gradle command could not pack those kind of Airflow Java SDK specific metadata, so it would be better to provide a gradle extension for users to compile the "fullly-Airflow compatible" jars.

---

We should wait until the AIP for "Defining Dag in Foreign-Lang" pass.

[uranusjr]

We don’t need this for now since it’s only useful for Java DAGs. Stub tasks are routed by the coordinator.