desi_extract_spectra: allow MASK and READNOISE to be optional

This came up while trying to apply DESI code to BOSS data: it would be handy to make the MASK and READNOISE extensions to be optional for desi_extract_spectra. The mask would default to 0 (nothing masked, allowing IVAR=0 to capture anything that should be masked), and a missing READNOISE HDU would fall back to looking for RDNOISE keywords (possibly RDNOISEn for different amps).

Start at the desi_extract_spectra entry point and inspect how the MASK and READNOISE extensions are currently required. Implement the requested optional behavior, including the mask default and RDNOISE keyword fallback, then run the existing extraction tests or relevant pipeline checks to confirm BOSS-style inputs work.

Written by the indexing model from the issue text.