[#596][FOLLOWUP] Index data support offheap read (#852)

### What changes were proposed in this pull request?
We support to read off heap data in the #806, we support to read index in this pr.

### Why are the changes needed?
#596 follow up pr

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?
GA passed.

storage/src/main/java/org/apache/uniffle/storage/handler/impl/HdfsShuffleReadHandler.java

@@ -84,18 +84,22 @@ public HdfsShuffleReadHandler(
   protected ShuffleIndexResult readShuffleIndex() {
     long start = System.currentTimeMillis();
     try {
-      byte[] indexData = indexReader.read();
-      int segmentNumber = indexData.length / FileBasedShuffleSegment.SEGMENT_SIZE;
+      ByteBuffer indexData = null;
+      if (offHeapEnabled) {
+        indexData = indexReader.readAsByteBuffer();
+      } else {
+        indexData = ByteBuffer.wrap(indexReader.read());
+      }
+      int indexDataLength = indexData.limit() - indexData.position();
+      int segmentNumber = indexDataLength / FileBasedShuffleSegment.SEGMENT_SIZE;
       int expectedLen = segmentNumber * FileBasedShuffleSegment.SEGMENT_SIZE;
-      if (indexData.length != expectedLen) {
+      if (indexDataLength != expectedLen) {
         LOG.warn("Maybe the index file: {} is being written due to the shuffle-buffer flushing.", filePrefix);
-        byte[] indexNewData = new byte[expectedLen];
-        System.arraycopy(indexData, 0, indexNewData, 0, expectedLen);
-        indexData = indexNewData;
+        indexData.limit(expectedLen);
       }
       long dateFileLen = getDataFileLen();
       LOG.info("Read index files {}.index for {} ms", filePrefix, System.currentTimeMillis() - start);
-      return new ShuffleIndexResult(ByteBuffer.wrap(indexData), dateFileLen);
+      return new ShuffleIndexResult(indexData, dateFileLen);
     } catch (Exception e) {
       LOG.info("Fail to read index files {}.index", filePrefix, e);
     }