Mock IMDSv2

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/configuring-instance-metadata-service.html

Can't see a good way to force using Version 1 in things like `awscli` right now, would be nice to have some minimum viable functionality for Version 2 in here to cover that off. From what I can tell, probably just the `.../api/token` namespace, plus the use of `PUT`?