cmake: Teach -E capabilities to report supported fileapi requests

Fixes: #19339

Source/cmFileAPI.h

@@ -36,6 +36,9 @@ public:
       and holding the original object.  Other JSON types are unchanged.  */
   Json::Value MaybeJsonFile(Json::Value in, std::string const& prefix);
 
+  /** Report file-api capabilities for cmake -E capabilities.  */
+  static Json::Value ReportCapabilities();
+
 private:
   cmake* CMakeInstance;