Support optional checksum for reading & writing files

[Chip Senkbeil]

A feature that would be handy is to support an optional field that is a checksum. This would be used in two ways

1. When reading a file's contents (bytes or text), we could include a field that says include_checksum. If true, then the server would compute a checksum of the file to send back that can be used to verify if the content has changed. Alternatively, we make this part of the metadata? Or have another command to do so. Not sure.
2. When writing a file's contents (bytes or text), we could include a field that says checksum. This would be the checksum of the file's contents presently. If provided and the file's checksum does not match, then the write would fail as the file had changed since the last attempt. Would be handy to ensure that we don't overwrite a remote file.

Also, we'd need to figure out if we want to specify a checksum type or just support a singular type like sha256.