[feature request] Support authentication for any kind of download

[stweil]

Some service providers (e.g. VL) use authentication for web access to digitisations that are still being processed and not ready for public access. Without authentication, such digitisations are not visible in OAI-PMH, and associated files such as METS or images are not accessible. Therefore, the use of OCR-D for such projects requires support for authentication.