[EKS] [request]: Support RFC6598 CG-NAT CIDR ranges for serviceIpv4Cidr

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**Tell us about your request**
We would like the Service CIDR range to be able to utilise RFC6598 address space (100.64.0.0/10)

**Which service(s) is this request for?**
EKS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Currently the CreateCluster action supports serviceIpv4Cidr in RFC1918 CIDR ranges, is there any reason why this can't be extended to include the Carrier-Grade NAT (CGNAT) range from RFC6598? Most networks make extensive use of RFC1918 address space, by being able to use the CGNAT CIDR ranges (e.g. 100.64.0.0/10) this would simplify address space reservation in our (and many other) organisations.

**Are you currently working around this issue?**
We are currently reserving ranges from RFC1918 but this may cause us a problem in the future.

Start with the EKS CreateCluster action’s serviceIpv4Cidr constraints and compare the supported RFC1918 ranges with RFC6598, including 100.64.0.0/10. The issue names no repository file, test, or implementation entry point; done would require confirmed EKS support for CGNAT service CIDR ranges and corresponding public behavior or documentation.

Written by the indexing model from the issue text.