DATAMONGO-1600 - Make GraphLookupOperationBuilder public.

Make GraphLookupOperationBuilder public so it can be used in types outside the aggregation package.

Author: mp911de

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/GraphLookupOperation.java

@@ -332,7 +332,7 @@ public GraphLookupOperationBuilder connectTo(String fieldName) {
 	/**
 	 * @author Mark Paluch
 	 */
-	static final class GraphLookupOperationBuilder {
+	public static final class GraphLookupOperationBuilder {
 
 		private final String from;
 		private final List<Object> startWith;