// ExecuteWorkflow starts a workflow execution and returns a WorkflowRun instance or error
		//
		// This can be used to start a workflow using a function reference or workflow type name.
		// Either by
		//     ExecuteWorkflow(ctx, options, "workflowTypeName", arg1, arg2, arg3)
		//     or
		//     ExecuteWorkflow(ctx, options, workflowExecuteFn, arg1, arg2, arg3)
		// The errors it can return:
		//  - serviceerror.NamespaceNotFound, if namespace does not exist
		//  - serviceerror.InvalidArgument
		//  - serviceerror.Internal
		//  - serviceerror.Unavailable