Add support for UDFs

dbt has provided support since version 1.11 to define UDFs in both Python and SQL formats: https://docs.getdbt.com/docs/build/udfs

ClickHouse currently offers:
- SQL functions are already available: https://clickhouse.com/docs/sql-reference/statements/create/function
- Python functions are in private preview: https://clickhouse.com/docs/sql-reference/functions/udf