[flink] Change the access modifier of FileStoreLookupFunction#getRequ…

…ireCachedBucketIds (apache#3532)

paimon-flink/paimon-flink-common/src/main/java/org/apache/paimon/flink/lookup/FileStoreLookupFunction.java

@@ -94,6 +94,8 @@ public class FileStoreLookupFunction implements Serializable, Closeable {
     // timestamp when cache expires
     private transient long nextLoadTime;
 
+    protected FunctionContext functionContext;
+
     public FileStoreLookupFunction(
             Table table, int[] projection, int[] joinKeyIndex, @Nullable Predicate predicate) {
         TableScanUtils.streamingReadingValidate(table);
@@ -127,6 +129,7 @@ public FileStoreLookupFunction(
     }
 
     public void open(FunctionContext context) throws Exception {
+        this.functionContext = context;
         String tmpDirectory = getTmpDirectory(context);
         open(tmpDirectory);
     }
@@ -352,7 +355,7 @@ private static StreamingRuntimeContext extractStreamingRuntimeContext(Object run
      *
      * @return the set of bucket IDs to be cached
      */
-    private Set<Integer> getRequireCachedBucketIds() {
+    protected Set<Integer> getRequireCachedBucketIds() {
         // TODO: Implement the method when Flink support bucket shuffle for lookup join.
         return null;
     }