Load knn vectors format with mmapfs (#78724)

Before the format used niofs. The current knn vectors implementation is based on
the HNSW algorithm, which is designed for the case where the graph and vectors
are be held in memory. Switching to mmapfs from niofs made a huge difference in
ANN benchmarks, speeding up some searches over 3x.

Author: jtibshirani

server/src/main/java/org/elasticsearch/index/store/LuceneFilesExtensions.java

@@ -69,8 +69,8 @@ public enum LuceneFilesExtensions {
     TVM("tvm", "Term Vector Metadata", true, false),
     TVX("tvx", "Term Vector Index", false, false),
     // kNN vectors format
-    VEC("vec", "Vector Data", false, false),
-    VEX("vex", "Vector Index", false, false),
+    VEC("vec", "Vector Data", false, true),
+    VEX("vex", "Vector Index", false, true),
     VEM("vem", "Vector Metadata", true, false);
 
     /**