add band to GainModel schema

Miri gain files on CRDS now contain the BAND keyword.

This PR adds a reference to keyword_band.schema to gain.schema.

It is not clear to me if at this point the miri gain files should be handled by a more specific schema for the following reasons:

https://jira.stsci.edu/browse/CRDS-730 mentions a range of possible values, these are a subset but not a match of those defined inkeyword_band.schema (one example keyword_band.schema lists MEDIUM which is not listed in the ticket)
my cursory understanding is that gain.schema, used for GainModel, is used for all gain reference files on crds. There are gain files for: fgs, miri, nircam, niriss and nirspec. Not all of these use the BAND keyword and not all of these use the FILTER keyword. Although apparently not required by the schema this does mean that adding a ref to keyword_band.schema means the GainModel instances for all of these files will now have a model.meta.instrument.band attribute

Checklist

spacetelescope / stdatamodels