add Flysystem Checksums support

[Gummibeer]

With Flysystem v3.7 checksum support was added: https://flysystem.thephpleague.com/docs/usage/checksums/
The Dropbox API provides a content_hash key - so it would be possible to add support for the new checksum method.
https://www.dropbox.com/developers/reference/content-hash

Would be great - not only to validate uploaded files match the pre-upload one but also to implement eTag for streamed responses for example.