Read only k8s group readonly

[ni9hty]

This diff adds a new EKS access entry for a ReadOnly IAM role. The new EKSAccessEntryReadOnlyAuth resource grants the ReadOnly role access to the EKS cluster with the same zalando:readonly Kubernetes group permissions as the existing access entry above it.

Key points:

• Creates access for arn:aws:iam::${AWS::AccountId}:role/ReadOnly
• Uses STANDARD access entry type
• Maps to zalando:readonly Kubernetes group
• Follows the same pattern as the existing access entry