feat: add ignore nulls options to concat function (#605)

This PR adds an `null_handling ` option to the concat function.

Some engines have two versions of the concat function. Where one ignores
null arguments.

example:
Postgres has the `||` operator which doesn't ignore nulls and `concat`
which does ignore nulls


https://www.postgresqltutorial.com/postgresql-string-functions/postgresql-concat-function/

extensions/functions_string.yaml

@@ -3,19 +3,30 @@
 scalar_functions:
   -
     name: concat
-    description: Concatenate strings.
+    description: >-
+      Concatenate strings.
+
+      The `null_handling` option determines whether or not null values will be recognized by the function.
+      If `null_handling` is set to `IGNORE_NULLS`, null value arguments will be ignored when strings are concatenated.
+      If set to `ACCEPT_NULLS`, the result will be null if any argument passed to the concat function is null.
     impls:
       - args:
           - value: "varchar<L1>"
             name: "input"
         variadic:
           min: 1
+        options:
+          null_handling:
+            values: [ IGNORE_NULLS, ACCEPT_NULLS ]
         return: "varchar<L1>"
       - args:
           - value: "string"
             name: "input"
         variadic:
           min: 1
+        options:
+          null_handling:
+            values: [ IGNORE_NULLS, ACCEPT_NULLS ]
         return: "string"
   -
     name: like