Add support for file-based ByteSteam (#412)

* Add support for file-based ByteSteam

This commit adds support for creating a `ByteStream` from a path or from
a file using `ReaderStream` as the transport mechanism.

In the long term, the implementation details may change significantly, however the external API,
"from_path", is expected to be stable.

* Inner does not need to be pub crate

* Show doc feature hints

* Ensure SdkBody is Send + Sync