[Java][FlexBuffers] Make FlexBuffersBuilder reusable by adding clear() (#5889) (#5890)

java/com/google/flatbuffers/FlexBuffersBuilder.java

@@ -151,6 +151,17 @@ public class FlexBuffersBuilder {
         this(bb, BUILDER_FLAG_SHARE_KEYS);
     }
 
+    /**
+     * Reset the FlexBuffersBuilder by purging all data that it holds.
+     */
+    public void clear(){
+        bb.clear();
+        stack.clear();
+        keyPool.clear();
+        stringPool.clear();
+        finished = false;
+    }
+
     /**
      * Return `ByteBuffer` containing FlexBuffer message. {@code #finish()} must be called before calling this
      * function otherwise an assert will trigger.