AWS::EC2::VPCCidrBlock - Enable extraction of CIDR blocks

## 1. Title
AWS::EC2::VPCCidrBlock - add CIDRv4 and CIDRv6 attributes

## 2. Scope of request
Need to be able to query the IPv4 and IPv6 CIDR ranges from a AWS::EC2::VPCCidrBlock. Currently, only the association itself is returned. The IPv6 return is especially important, and AWS generates this prefix internally and so there's no way of knowing until resource create time.

## 3. Expected behavior

GetAtt calls for the Ipv4 and IPv6 prefixes returns a valid CIDR from the VPC.

## 4. Suggest specific test cases

Nothing specific, just generate a new CIDR association and see if the CIDR's are correct.

## 5. Helpful Links to speed up research and evaluation

## 6. Category (required) - Will help with tagging and be easier to find by other users to +1

4. Networking & Content (VPC, Route53, API GW,...)

## 7. Any additional context (optional)