aggregate_fcts: Use per-type comparators for dynamic types
aggregate_fcts: Use per-type comparators for dynamic types

For collections and UDTs the `MIN()` and `MAX()` functions are
generated on the fly. Until now they worked by comparing just the
byte representations of arguments.

This patch uses specific per-type comparators to provide semantically
sensible, dynamically created aggregates.

Fixes #6768

scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla