[FEA]: Allow instances with interfaces from multiple VPCs in the REST API


 
- New `multi_vpc_instance` flag in site.config to toggle this.
- If enabled vpc-prefix <-> instance.vpc is only required for the PF interface (or PF of the first DPU if there is one)
- Optional `secondaryVpcIds` field added to Instance model
- On output, `secondaryVpcIds` can be populated based on interface data.