feat: add ability to check for existence of a field

packages/core/src/protocol/search/filters/value.ts

@@ -33,6 +33,8 @@ export interface SCValueFilterArguments extends SCSearchAbstractFilterArguments
 
   /**
    * Value to filter. One or more values has to match the field exactly.
+   *
+   * Leaving the value out will check if the field exists.
    */
-  value: string | string[];
+  value?: string | string[];
 }