Make HostColumnVector.getRefCount public (#13934)

This is a small PR to make HostColumnVector.getRefCount public. 

This is useful in figuring out spillability for a `HostColumnVector` (plugin issue: NVIDIA/spark-rapids#8882)

Authors:
  - Alessandro Bellina (https://github.com/abellina)

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

URL: #13934

java/src/main/java/ai/rapids/cudf/HostColumnVector.java

@@ -199,7 +199,7 @@ private synchronized HostColumnVector incRefCountInternal(boolean isFirstTime) {
   /**
    * Returns this column's current refcount
    */
-  synchronized int getRefCount() {
+  public synchronized int getRefCount() {
     return refCount;
   }