Add function to append ".keyword" to field name

Added a static method 'asKeyword' in Query.php. This appends a ".keyword" suffix to the field name, aiding in sorting and aggregating on keyword fields.
It can be used like `$field = Query::asKeyword($field);`.
This makes some querying code easier to read.

src/Query.php

@@ -22,6 +22,7 @@
 use Matchory\Elasticsearch\Interfaces\ConnectionInterface;
 use function count;
 use function json_encode;
+use function rtrim;
 use const JSON_THROW_ON_ERROR;
 
 /**
@@ -203,6 +204,18 @@ public function __construct(
         $this->model = $model ?? new Model();
     }
 
+    /**
+     * Adds a ".keyword" suffix to the given field name. This is useful for
+     * sorting and aggregating on keyword fields.
+     *
+     * @param string $field
+     * @return string
+     */
+    public static function asKeyword(string $field): string
+    {
+        return rtrim($field, '.') . '.keyword';
+    }
+
     /**
      * Retrieves the underlying Elasticsearch connection.
      *