This change adds a new Check RPC method on the provider interface,
permitting resource providers to perform arbitrary verification on
the values of properties. This is useful for validating things
that might be difficult to express in the type system, and it runs
before *any* modifications are run (so failures can be caight early
before it's too late). My favorite motivating example is verifying
that an AWS EC2 instance's AMI is available within the target region.
This resolvespulumi/coconut#107, although we aren't using this
in any resource providers just yet. I'll add a work item now for that...