Add `format` to Spectra table

add `specutils_format` column to Spectra table which could then be passed to specutils.read. Could really simplify LOTS of things and encourage more formats to be added to specutils.

Let's beta test in SIMPLE:
https://github.com/SIMPLE-AstroDB/SIMPLE-db/issues/656

Start by locating the Spectra table schema and review the linked SIMPLE issue 656 for the beta-test context. Confirm where the new specutils_format value would be supplied to specutils.read; done means the column exists and the SIMPLE beta test can use it.

Written by the indexing model from the issue text.