Move IAM API types from /cmd/clusterawsadm/api to /api in v1alpha4

[randomvariable]

/kind feature

Describe the solution you'd like
[A clear and concise description of what you want to happen.]
Since IAM types are now consumed by the controllers, the API group should "graduate" from /cmd/clusterawsadm/api/iam/v1alpha1 to /api/v1alpha4/iam

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• Cluster-api-provider-aws version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

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