Add CUSTOM ami_type support.

[Ramyak]

Is your request related to a new offering from AWS?

No.

Is your request related to a problem? Please describe.

https://docs.aws.amazon.com/eks/latest/APIReference/API_Nodegroup.html#AmazonEKS-Type-Nodegroup-amiType CUSTOM ami_type.

1. Can the module support CUSTOM AMI type? https://github.com/terraform-aws-modules/terraform-aws-eks/blob/v20.17.2/modules/self-managed-node-group/main.tf#L13-L27

2. For CUSTOM AMI type (self managed node group), can we not assume that the AMI will understand content_type = "application/node.eks.aws"

Describe the solution you'd like.

• Launch custom AMI type, that does not have nodeadm installed.

Describe alternatives you've considered.

Additional context