SQL UDFs and Procedural language

We will need to support a way to write custom user-defined functions https://www.postgresql.org/docs/current/xfunc-sql.html. These will work in a similar way to the current check operators. Additionally, some rules (CG0019) require the use of metadata to create queries. In order to do this, we will require a way of supporting a procedural language. We may be able to support user defined procedures the same way we support user defined functions.
- Postgresql supports 4 procedural languages. https://www.postgresql.org/docs/current/xplang.html. We will need to choose between:
- a sql-like language in the rule yaml
- a python language (PL/Python) in an external folder or repo. This would be similar to how we handle operations and operators