Implement SortOptionsVariant in FieldSort

CHANGELOG.md

@@ -51,6 +51,7 @@ This section is for maintaining a changelog for all breaking changes for the cli
 ### Removed
 
 ### Fixed
+- Fixed an issue where `FieldSort` was not implementing `SortOptionsVariant` ([#1323](https://github.com/opensearch-project/opensearch-java/pull/1323))
 
 ### Security
 

java-client/src/main/java/org/opensearch/client/opensearch/_types/FieldSort.java

@@ -49,7 +49,7 @@
 // typedef: _types.FieldSort
 
 @JsonpDeserializable
-public class FieldSort implements PlainJsonSerializable {
+public class FieldSort implements SortOptionsVariant, PlainJsonSerializable {
     // Single key dictionary
     private final String field;
 
@@ -94,6 +94,14 @@ public static FieldSort of(Function<Builder, ObjectBuilder<FieldSort>> fn) {
         return fn.apply(new Builder()).build();
     }
 
+    /**
+     * SortOptions variant kind.
+     */
+    @Override
+    public SortOptions.Kind _sortOptionsKind() {
+        return SortOptions.Kind.Field;
+    }
+
     /**
      * Required - The target field
      */