Add child workflow implementation under the new worker config

[chopincode]

To start a parent workflow, try cadence --env development --domain samples-domain workflow start --workflow_type ParentWorkflow::getGreetingInParent --et 60 --tl cadence-samples-worker --input '{"message":"Uber"}'

To start the child workflow in standalone style, try cadence --env development --domain samples-domain workflow start --workflow_type ChildWorkflow::greetInChild --et 60 --tl cadence-samples-worker --input '"Uber"'