Support index-field to create a GIN index

load_queryables() always sets property_index_type = 'BTREE' for any field passed to --index-field. It seems there's no way to ask for anything else.

That's a problem for array properties as the only operators STAC defines for arrays (a_overlaps/a_contains/a_equals → &&/@>/<@) need a GIN index to be fast.

It seems indexdef() already builds the index from whatever's in property_index_type with no check on the value. So GIN should already works fine if that column is set to 'GIN'.

Would it make sense to:

• let --index-field take a type, like monty:country_codes:gin (BTREE stays the default)?
• or just default array fields to GIN automatically, since BTREE doesn't them anyway?

Happy to open a PR either way, if one of these sounds right.

Start by reading load_queryables() and indexdef(), the entry points named in the issue, and trace how --index-field populates property_index_type. Clarify whether the change should accept an explicit GIN type or choose GIN for array fields. Done means array-property indexes use GIN while the existing BTREE default remains intact.