Streaming API (#420)

This PR adds an API for streaming data from workflows.

```python
def write_stream(cls, key: str, value: Any) -> None:
    """
    Write a value to a stream.

    Args:
        key(str): The stream key / name within the workflow
        value(Any): A serializable value to write to the stream

    """

def close_stream(cls, key: str) -> None:
    """
    Close a stream.

    Args:
        key(str): The stream key / name within the workflow

    """

def read_stream(cls, workflow_id: str, key: str) -> Generator[Any, Any, None]:
    """
    Read values from a stream as a generator.

    This function reads values from a stream identified by the workflow_id and key,
    yielding each value in order until the stream is closed or the workflow terminates.

    Args:
        workflow_id(str): The workflow instance ID that owns the stream
        key(str): The stream key / name within the workflow

    Yields:
        Any: Each value in the stream until the stream is closed

    """
```

All methods also have async equivalents.

Streams are append-only and immutable and can be read from anywhere.

Streams can be written to from a workflow or its steps. Writing to a
stream from a workflow is done with exactly-once semantics. Writing to a
stream from a step is at-least-once; if a step fails and is retried, it
may write to the stream multiple times.