Break up DDRProcessor.java

It has grown to such size that breaking it up is long overdue anyway,
but the immediate impetus was to do a trial javadoc run checking
everything (--show-module-contents all --show-packages-all
--show-types private --show-members private). I have no intention of
changing the doc build options to build all that as a matter of course,
but it ought to be possible, so a run with those settings is useful to
identify and fix any javadoc-reported errors that cause the generation
to fail, as opposed to mere warnings.

Javadoc was immediately displeased by the multi-class compilation unit
here.

Author: jcflack

pljava-api/src/main/java/org/postgresql/pljava/annotation/processing/Commentable.java

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2016-2025 Tada AB and other contributors, as listed below.
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the The BSD 3-Clause License
+ * which accompanies this distribution, and is available at
+ * http://opensource.org/licenses/BSD-3-Clause
+ *
+ * Contributors:
+ *   Purdue University
+ *   Chapman Flack
+ */
+package org.postgresql.pljava.annotation.processing;
+
+import javax.lang.model.element.Element;
+
+interface Commentable
+{
+	public String comment();
+	public void setComment( Object o, boolean explicit, Element e);
+	public String derivedComment( Element e);
+}