Generalize index support in network support function

Generalize index support in network support function

The network (inet) support functions currently only supported a
hardcoded btree operator family.  With the generalized compare type
facility, we can generalize this to support any operator family from
any index type that supports the required operators.

Author: Mark Dilger <mark.dilger@enterprisedb.com>
Co-authored-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://www.postgresql.org/message-id/flat/E72EAA49-354D-4C2E-8EB9-255197F55330@enterprisedb.com