fix: add missing Execution export

The Execution class needs to be exported by gooddata_sdk,
so that users can hold a reference to it in a type-safe way.

JIRA: CQ-1124
risk: low

Author: no23reason

gooddata-sdk/gooddata_sdk/__init__.py

@@ -231,6 +231,7 @@
 from gooddata_sdk.compute.model.base import ExecModelEntity, ObjId
 from gooddata_sdk.compute.model.execution import (
     BareExecutionResponse,
+    Execution,
     ExecutionDefinition,
     ExecutionResponse,
     ExecutionResult,