Expose JavaAPI for getting the filter policy of a BlockBasedTableConfig (#5186)

Summary:
I would like to be able to read out the current Filter that has been set (or not) for a BlockBasedTableConfig. Added one public method to BlockBasedTableConfig:

public Filter filterPolicy() {
    return filterPolicy;
}
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5186

Differential Revision: D14921415

Pulled By: siying

fbshipit-source-id: 2a63c8685480197862b49fc48916c757cd6daf95

java/src/main/java/org/rocksdb/BlockBasedTableConfig.java

@@ -479,6 +479,15 @@ public class BlockBasedTableConfig extends TableFormatConfig {
     return this;
   }
 
+  /**
+   * Get the filter policy.
+   *
+   * @return the current filter policy.
+   */
+  public Filter filterPolicy() {
+    return filterPolicy;
+  }
+
   /**
    * Use the specified filter policy to reduce disk reads.
    *