[Workflows] Rename callSubWorkflow to callChildWorkflow

[olitomlinson]

The Java SDK uses the method name callSubWorkflow rather than callChildWorkflow in the workflow context

The Dapr documentation refers to child workflows , so I would recommend that the Java SDK change the method name to callChildWorkflow for consistency.

[salaboy]

@olitomlinson ok.. PR created.. I will follow the tests to see if I am missing something.
@artursouza do you know if we need to deprecate the old methods? Do you know how to create a release note for this change? It is a simple replace to upgrade to this new signature.