Support modify subnet attribute for AWS VPC

The only thing modify subnet attribute can modify is the MapPublicIpOnLaunch flag.

I chose to have the option passed be  `MapPublicIpOnLaunch` rather than `MapPublicIpOnLaunch.Value` - although that's the parameter name the api actually required it felt a bit over the top so i thought it was ok to  handle that for callers of the request