Sfn-workflow-client

Sets the default value of `AWS_ACCOUNT_ID` using boto3's STS client's `get_caller_identity` method.

Adds the ability to pass the Step Functions endpoint URL into the `Workflow` constructor. This is handy for switching between API backends in tests.

- Trigger new executions
- Query for state machine execution status
- Wait for an execution to complete
- Fetch execution history