fix: make ConstantKeywordFieldType protected

Signed-off-by: Mohammad Hasnain <hasnain2808@gmail.com>

Author: hasnain2808

server/src/main/java/org/opensearch/index/mapper/ConstantKeywordFieldMapper.java

@@ -98,7 +98,7 @@ public ConstantKeywordFieldMapper build(BuilderContext context) {
      * @opensearch.internal
      */
     @PublicApi(since = "2.14.0")
-    static final class ConstantKeywordFieldType extends ConstantFieldType {
+    protected static final class ConstantKeywordFieldType extends ConstantFieldType {
 
         protected final String value;