Support marking AWSManagedControlPlane as externally managed

/kind feature

Describe the solution you'd like Support for AWSManagedControlPlane resources to be marked as externally managed through the cluster.x-k8s.io/managed-by: "<name-of-system>" annotation to prevent reconciliation.

Anything else you would like to add: We have EKS clusters managed outside of the CAPI/CAPA ecosystem (via Terraform) but are using CAPI/CAPA to manage the life-cycling of Machines/Nodes (EKS Hybrid Nodes to be precise, another issue/PR/discussion incoming). In this situation AWSManagedControlPlane resources are used a read-only stub of the information needed for the bootstrap controller to do its job.

This concept is already supported for AWSCluster resources as documented here. I am keen to contribute a PR for this if the request has supported 👍 .

Environment:

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