fix: handle valid YAML extension types

Closes #594

Author: benbellick

core/src/main/java/io/substrait/extension/SimpleExtension.java

@@ -535,12 +535,19 @@ public abstract static class Type {
 
     public abstract String name();
 
+    public abstract Optional<String> description();
+
     @JacksonInject(SimpleExtension.URN_LOCATOR_KEY)
     public abstract String urn();
 
     // TODO: Handle conversion of structure object to Named Struct representation
     protected abstract Optional<Object> structure();
 
+    // TODO: Properly handle parameters
+    protected abstract Optional<List<Object>> parameters();
+
+    protected abstract Optional<Boolean> variadic();
+
     public TypeAnchor getAnchor() {
       return anchorSupplier.get();
     }

core/src/test/resources/extensions/custom_extensions.yaml

@@ -3,6 +3,10 @@
 urn: extension:test:custom_extensions
 types:
   - name: "customType1"
+    description: >-
+      some description
+    variadic: true
+    parameters: []
   - name: "customType2"
     structure:
       field1: i32