[docker-py] Add exec_start overload

[roberfi]

This new overload is needed for a wrapping function like:

def my_function(
    obj: ExecApiMixin,
    exec_id: str,
    detach: bool = False,
    tty: bool = False,
    stream: bool = False,
    socket: bool = False,
    demux: bool = False,
) -> None:
    obj.exec_start(exec_id, detach, tty, stream, socket, demux)

If this new overload is not present, mypy will give the following error:

No overload variant of "exec_start" of "ExecApiMixin" matches argument types "str", "bool", "bool", "bool", "bool", "bool"

Besides, return type when stream=True (False for the rest) is CancellableStream[bytes] instead of CancellableStream[str] and bytes instead of str if everything is False.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉