GH-37246: [Java] expose VectorAppender class to offer support to appe…

…nd vector values (#37247)

### Rationale for this change

- To close #37246 

### What changes are included in this PR?

- Expose VectorAppender class to offer support to append vector values

### Are these changes tested?

- By TestVectorAppender

### Are there any user-facing changes?

- No
* Closes: #37246

Authored-by: david dali susanibar arce <davi.sarces@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>

java/vector/src/main/java/org/apache/arrow/vector/util/VectorAppender.java

@@ -43,7 +43,7 @@
 /**
  * Utility to append two vectors together.
  */
-class VectorAppender implements VectorVisitor<ValueVector, Void> {
+public class VectorAppender implements VectorVisitor<ValueVector, Void> {
 
   /**
    * The targetVector to be appended.
@@ -56,7 +56,7 @@ class VectorAppender implements VectorVisitor<ValueVector, Void> {
    * Constructs a new targetVector appender, with the given targetVector.
    * @param targetVector the targetVector to be appended.
    */
-  VectorAppender(ValueVector targetVector) {
+  public VectorAppender(ValueVector targetVector) {
     this.targetVector = targetVector;
     typeVisitor = new TypeEqualsVisitor(targetVector, false, true);
   }