Rename workflow to subflow

[mcruzdev]

As is

Today the DSL looks weird when we need to call a workflow inside a workflow:

    Workflow wf =
        WorkflowBuilder.workflow("parent", "ns", "1")
            .tasks(
                workflow(
                    workflow("child.ns", "child-flow", "2.3.4")
                        .input("id", 99)
                        .await(false)
                        .returnType(RunTaskConfiguration.ProcessReturnType.NONE)))
            .build();

We have workflow(workflow it sounds weird.

Proposal

To change workflow to subflow to have a more clear and intuitive DSL.