create_function migration helper needs "language" and "immutable" options

[jwoertink]

avram/src/avram/migrator/statement_helpers.cr

Line 67 in a067982

def create_function(name : String, body : String, returns : String = "trigger")

When creating a new function you may want to specify the language the function uses or if the function is immutable.

CREATE OR REPLACE FUNCTION public.some_function(thing boolean)
         RETURNS boolean
         LANGUAGE plpgsql
         IMMUTABLE
AS $$
BEGIN
....
END
$$

For defaults we could use "VOLATILE" for immutable and sql for the language.