MultiBucketsAggregation.Bucket should not extend Writeable (#24216)

The MultiBucketsAggregation.Bucket interface extends Writeable, forcing
all implementation classes to implement writeTo(). This commit removes
the Writeable from the interface and move it down to the InternalBucket
implementation.

Author: tlrx

core/src/main/java/org/elasticsearch/search/aggregations/InternalMultiBucketAggregation.java

@@ -20,6 +20,7 @@
 package org.elasticsearch.search.aggregations;
 
 import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.common.io.stream.Writeable;
 import org.elasticsearch.search.aggregations.bucket.MultiBucketsAggregation;
 import org.elasticsearch.search.aggregations.pipeline.PipelineAggregator;
 
@@ -81,7 +82,7 @@ public Object getProperty(List<String> path) {
         }
     }
 
-    public abstract static class InternalBucket implements Bucket {
+    public abstract static class InternalBucket implements Bucket, Writeable {
 
         public Object getProperty(String containingAggName, List<String> path) {
             if (path.isEmpty()) {

core/src/main/java/org/elasticsearch/search/aggregations/bucket/MultiBucketsAggregation.java

@@ -19,7 +19,6 @@
 
 package org.elasticsearch.search.aggregations.bucket;
 
-import org.elasticsearch.common.io.stream.Writeable;
 import org.elasticsearch.common.util.Comparators;
 import org.elasticsearch.common.xcontent.ToXContent;
 import org.elasticsearch.search.aggregations.Aggregation;
@@ -37,7 +36,7 @@ public interface MultiBucketsAggregation extends Aggregation {
      * A bucket represents a criteria to which all documents that fall in it adhere to. It is also uniquely identified
      * by a key, and can potentially hold sub-aggregations computed over all documents in it.
      */
-    interface Bucket extends HasAggregations, ToXContent, Writeable {
+    interface Bucket extends HasAggregations, ToXContent {
         /**
          * @return The key associated with the bucket
          */