[SQL] Support for user-defined functions written in SQL

[mihaibudiu]

Is this a user-visible change (yes/no): yes

Fixes #1135

This should always work, and doesn't require support from the pipeline manager.
Here is an example from the tests:

                CREATE FUNCTION contains_number(str VARCHAR NOT NULL, value INTEGER)
                RETURNS BOOLEAN NOT NULL
                AS (str LIKE ('%' || COALESCE(CAST(value AS VARCHAR), 'NULL') || '%'));

[lalithsuresh]

Can the function run queries?

[mihaibudiu]

These have to be pure expressions. Table functions maybe later