udt: Add is_user_type function to abstract_type
udt: Add is_user_type function to abstract_type

This patch adds a function to identify a given abstract_type as a
user_type_impl.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

avikivity/scylla,raphaelsc/scylla,raphaelsc/scylla,kjniemi/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,kjniemi/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,duarten/scylla,raphaelsc/scylla