db:schema:load fails if a user defined function already exists

[danielwestendorf]

In a rails app, with existing Clickhouse schema w/a function defined, a rails db:schema:load will not always succeed; this is because the function may already exist.

Code: 609. DB::Exception: User-defined function 'uuid7ToDateTime' already exists. (FUNCTION_ALREADY_EXISTS) (version 23.9.6.20 (official build))

This can be addressed in at least two ways:

1. Guard the CREATE FUNCTION statement with IF NOT EXISTS; this would prevent trying to create the function, however, it also does not guarantee that the function is up-to-date with the schema
2. Use CREATE OR REPLACE FUNCTION instead. This has the benefit of ensuring the function is up-to-date with the schema