WIP: add knn_vector as recognized MappingType in OpenSearchDataType

Register knn_vector as KnnVector mapping to ExprCoreType.ARRAY so that
knn_vector fields appear in SELECT * results and are projectable as
array-of-float columns.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mengweieric

opensearch/src/main/java/org/opensearch/sql/opensearch/data/type/OpenSearchDataType.java

@@ -43,7 +43,8 @@ public enum MappingType {
     ScaledFloat("scaled_float", ExprCoreType.DOUBLE),
     Double("double", ExprCoreType.DOUBLE),
     Boolean("boolean", ExprCoreType.BOOLEAN),
-    Alias("alias", ExprCoreType.UNKNOWN);
+    Alias("alias", ExprCoreType.UNKNOWN),
+    KnnVector("knn_vector", ExprCoreType.ARRAY);
     // TODO: ranges, geo shape, point, shape
 
     private final String name;