Respect RowDescription.dataTypeModifier where appropriate

[gwynne]

The dataTypeModifier, corresponding to pg_attribute.atttypmod in the system tables (ref: https://www.postgresql.org/docs/15/catalog-pg-attribute.html), is currently completely ignored. It is currently only defined for the built-in types varbit and varchar (and any custom types that may need it), but the value is potentially meaningful and should be available during decoding (e.g. provided by PostgresCell etc.)