Workflow Status Enum for Autocompletion

A user asks that we change
```
class WorkflowStatus:
    ...
    # The workflow status. Must be one of ENQUEUED, PENDING, SUCCESS, ERROR, CANCELLED, or RETRIES_EXCEEDED
    status: str
```

to
```
class WorkflowStatus:
    ...
    # The workflow status.
    status: Literal["ENQUEUED", "PENDING", "SUCCESS", "ERROR", "CANCELLED", "RETRIES_EXCEEDED"]
```