feat: add null input handling options for nth_value

nth_value can be used in place of e.g. first() in Spark
but it's missing an option for whether to ignore nulls in
input or not

extensions/functions_aggregate_generic.yaml

@@ -32,6 +32,11 @@ aggregate_functions:
     impls:
       - args:
           - name: x
-            value: any
+            value: any1
+        options:
+          ignore_nulls:
+            values: [ "TRUE", "FALSE" ]
         nullability: DECLARED_OUTPUT
-        return: any?
+        decomposable: MANY
+        intermediate: any1?
+        return: any1?