The recent advancements in cellular Vehicle-to-everything (C-V2X) and edge computing paradigms foster novel use cases for connected and automated vehicles that come with strict performance requirements. These guarantees can be provided by network slicing, which is currently enabled by the virtualization and cloudification of mobile networks, following the 5G/6G core service-based approach and the cloud-native Open Radio Access Network (O-RAN) principles. However, a significant challenge arises when considering the movement of end users: ensuring network slice continuity as they transition between different network operators. In this paper, we introduce a novel cloud-native orchestration framework for network slice federation that incorporates well-defined interfaces to exchange federated service and slice resource templates among operators. The proposed framework is fully compliant with i) standardized slice service models, and ii) GSMA efforts that are building the fundamental aspects for Edge Federation to allow the sharing of network resources across mobile operators. To validate our approach, we have designed and deployed a cloud-native federated 5G experimental platform. An extensive series of experiments have been carried out, revealing that the federation implementation directly influences federation time, and the operator's slice deployment strategy significantly impacts both infrastructure and end user performance.