Set default space type to L2 to support bwc

This change adds L2 space type fall back during query.
This prevents backwards compatibility issues for clusters
upgrading from ES 7.1 and 7.4. In those versions, space
type was not introduced as a field attribute yet.

src/main/java/org/opensearch/knn/index/KNNWeight.java

@@ -110,7 +110,8 @@ public Scorer scorer(LeafReaderContext context) throws IOException {
             } else {
                 String engineName = fieldInfo.attributes().getOrDefault(KNN_ENGINE, KNNEngine.NMSLIB.getName());
                 knnEngine = KNNEngine.getEngine(engineName);
-                spaceType = SpaceType.getSpace(fieldInfo.getAttribute(SPACE_TYPE));
+                String spaceTypeName = fieldInfo.attributes().getOrDefault(SPACE_TYPE, SpaceType.L2.getValue());
+                spaceType = SpaceType.getSpace(spaceTypeName);
             }
 
             /*