Implement a version of download_products that yields file-like/fits objects

For some use cases I find it awkward to be handling files and keeping track of directory structures to deal with datasets downloaded from mast. Might it be possible to have a version of `download_products` that returns e.g. `urllib` objects for everything and/or open `HDUList` objects for data products that are fits files?

This is also potentially an alternate way to solve #1150 - implementing this would leave the user free to apply their own directory structure or save the file in any arbitrary location with unique file names...

Start by reading the existing download_products entry point and the related discussion in issue #1150. Compare the requested urllib-style objects with open HDUList objects for FITS products, then clarify the API and ownership of downloaded resources before implementation. Done means a documented, tested approach that lets callers consume products without relying on the existing directory structure.

Written by the indexing model from the issue text.