Make Java AstNode public (rapidsai#8953)

This makes the Java `AstNode` base class public so applications can construct an AST using a common type for all AST nodes.

Authors:
  - Jason Lowe (https://github.com/jlowe)

Approvers:
  - Robert (Bobby) Evans (https://github.com/revans2)

URL: rapidsai#8953

java/src/main/java/ai/rapids/cudf/ast/AstNode.java

@@ -19,7 +19,7 @@
 import java.nio.ByteBuffer;
 
 /** Base class of every node in an AST */
-abstract class AstNode {
+public abstract class AstNode {
   /**
    * Enumeration for the types of AST nodes that can appear in a serialized AST.
    * NOTE: This must be kept in sync with the `jni_serialized_node_type` in CompiledExpression.cpp!