WebDriver: Implement GET /session/{session id}/window/handles endpoint

Author: https://github.com/linusg
Commit: 6641c99c80
Pull-request: https://github.com/SerenityOS/serenity/pull/15704
Reviewed-by: https://github.com/TobyAsE ✅

Userland/Services/WebDriver/Session.h

@@ -48,6 +48,7 @@ public:
     ErrorOr<JsonValue, HttpError> get_title();
     ErrorOr<JsonValue, HttpError> get_window_handle();
     ErrorOr<void, Variant<HttpError, Error>> close_window();
+    ErrorOr<JsonValue, HttpError> get_window_handles() const;
     ErrorOr<JsonValue, HttpError> find_element(JsonValue const& payload);
     ErrorOr<JsonValue, HttpError> find_elements(JsonValue const& payload);
     ErrorOr<JsonValue, HttpError> find_element_from_element(JsonValue const& payload, StringView parameter_element_id);