WADO-RS: Skip file parsing for faster reads

[nickamzol]

Do we really need to parse every DICOM file before sending them to the client?
I think we can just pass through the raw data (Vec<u8>), unless we want to change some attributes (is there some kind of lazy parsing in dicom-rs so we do not need to parse the entire file?).

Maybe introduce a new config option validate: <false|true>, as validation of data before sending seems like a good idea?