Allow String workflow_id as first parameter for certain decider methods.

Allow signal_external_workflow_execution and request_cancel_external_workflow_execution to accept a String workflow id as the first parameter. This is consistent with the current documentation.

At runtime, this means that users of this code will be able to signal or cancel workflows simply with a (possibly deterministic) workflow id, and will nto have to call describe_workflow_execution to get the relevant run_id.

Note that for both of these decision types, the HTTP AWS API allows creating the decision without a run_id.