Add support to subflows on the Java DSL

[ricardozanini]

What would you like to be added:
Currently, users can't declare workflow calls from the Java DSL. See the language reference here: https://github.com/serverlessworkflow/specification/blob/main/dsl-reference.md#workflow-process

The runtime engine already supports calling another workflow definition, it's just a matter of supporting it on the Java DSL. Something like:

FuncDSL.workflow().tasks(workflow(myref))

Why is this needed:
To complete the Java DSL support for the language definition.