feat(isthmus): udf support for substrait<->calcite

# Conflicts:
#	isthmus/src/main/java/io/substrait/isthmus/SqlToSubstrait.java

Author: ZorinAnton

isthmus/src/test/resources/extensions/functions_string_custom.yaml

@@ -0,0 +1,11 @@
+%YAML 1.2
+---
+scalar_functions:
+  - name: "regexp_extract"
+    impls:
+      - args:
+          - name: "text"
+            value: string
+          - name: "pattern"
+            value: string
+        return: string