Add ByteStream abstraction (#333)

* Add ByteStream abstraction

Introduce `ByteStream`, a wrapper around SdkBody to provide easy-to-use abstractions around streaming binary data.

* Updates for CR feedback
- `data` is renamed to `collect`
- Added `into_inner` so that you can use it as a body directly if desired
- Removed unecessary impl for io::Error
- Added `From<SdkBody> for ByteStream
- Project to remove need for `Unpin` in `Inner`
- Forward relevant method in `Buf` implementation
- Use `try_next` in example

* Fix missed `data -> Collect`

* Show don't tell in examples