[Provisioning] Dynamically fetch instance types for packing

[bwagner5]

The provisioner needs to dynamically fetch possible ec2 instance types to pack pods on.

The current implementation is hardcodes the instance types and corresponding resource specs: https://github.com/awslabs/karpenter/blob/35b6517c8ffed6c81cbe9b2405e047462122dc79/pkg/cloudprovider/aws/packing/nodecapacity.go#L25-L75

Plan:

For now, I think it's sufficient to fetch instance types using ec2.DescribeInstanceTypes and populate nodeCapacity structs. Filtering based on pod specs will need to be implemented later.