Container for the annotation-based Dag-authoring API.

This class is not instantiated directly. Its nested annotations drive the BuilderProcessor annotation processor, which generates a *Builder class for each class annotated with Dag.

Example:

@Builder.Dag(id = "my_pipeline")
public class MyPipeline {

    @Builder.Task(id = "extract")
    public long extract(Client client) { ... }

    @Builder.Task(id = "transform")
    public long transform(Client client, @Builder.XCom(task = "extract") long extracted) { ... }
}

The processor generates MyPipelineBuilder.build(), which returns a fully wired-up Dag ready to add to a Bundle.