Make sibling pipeline agg ctor's protected (#42808)

SiblingPipelineAggregator is a public interfaces,
but the ctor was package-private.  These should be protected so that
plugin authors can extend and implement their own sibling pipeline agg.

server/src/main/java/org/elasticsearch/search/aggregations/pipeline/SiblingPipelineAggregator.java

@@ -36,7 +36,7 @@ import java.util.stream.Collectors;
 import java.util.stream.StreamSupport;
 
 public abstract class SiblingPipelineAggregator extends PipelineAggregator {
-    SiblingPipelineAggregator(String name, String[] bucketsPaths, Map<String, Object> metaData) {
+    protected SiblingPipelineAggregator(String name, String[] bucketsPaths, Map<String, Object> metaData) {
         super(name, bucketsPaths, metaData);
     }