GH-37863: [Java] Add typed getters for StructVector (#37916)

### Rationale for this change
Add methods for getting child vectors as specific vector subtypes
for convenience.

### What changes are included in this PR?
Add child getters which let the caller specify the target vector type to StructVector.

### Are these changes tested?
Yes.

### Are there any user-facing changes?
No.
* Closes: #37863

Authored-by: James Duong <duong.james@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>

Author: jduo

java/vector/src/main/java/org/apache/arrow/vector/complex/NonNullableStructVector.java

@@ -374,6 +374,18 @@ public ValueVector getVectorById(int id) {
     return getChildByOrdinal(id);
   }
 
+  /**
+   * Gets a child vector by ordinal position and casts to the specified class.
+   */
+  public <V extends ValueVector> V getVectorById(int id, Class<V> clazz) {
+    ValueVector untyped = getVectorById(id);
+    if (clazz.isInstance(untyped)) {
+      return clazz.cast(untyped);
+    }
+    throw new ClassCastException("Id " + id + " had the wrong type. Expected " + clazz.getCanonicalName() +
+        " but was " + untyped.getClass().getCanonicalName());
+  }
+
   @Override
   public void setValueCount(int valueCount) {
     for (final ValueVector v : getChildren()) {

java/vector/src/test/java/org/apache/arrow/vector/TestStructVector.java

@@ -282,4 +282,12 @@ public void testAddChildVectorsWithDuplicatedFieldNamesForConflictPolicyReplace(
     }
   }
 
+  @Test
+  public void testTypedGetters() {
+    try (final StructVector s1 = StructVector.empty("s1", allocator)) {
+      s1.addOrGet("struct_child", FieldType.nullable(MinorType.INT.getType()), IntVector.class);
+      assertEquals(IntVector.class, s1.getChild("struct_child", IntVector.class).getClass());
+      assertEquals(IntVector.class, s1.getVectorById(0, IntVector.class).getClass());
+    }
+  }
 }