Persist message list search type's sort option. (Graylog2#7759)

Before this change, the `sort` option for a message list search type was
not persisted. This lead to sorting configuration having no effect when
storing a search and executing it, only for the synchronous endpoint.

Fixes Graylog2#7758.

graylog2-server/src/main/java/org/graylog/plugins/views/search/searchtypes/MessageList.java

@@ -78,6 +78,7 @@ public abstract class MessageList implements SearchType {
     public abstract int offset();
 
     @Nullable
+    @JsonProperty
     public abstract List<Sort> sort();
 
     @JsonProperty