Allow to configure garbage collector using clusterawsadm

[Fedosin]

/kind feature

Describe the solution you'd like
Recently we added a possibility to configure garbage collector for AWS clusters: #4466 To do so we need to set an annotation for the related AWSCluster object. It's not convenient to do it manually, so I'd like to let clusterawsadm cli tool to do it for me.

It should be implemented under clusterawsadm gc configure section, where users can add or remove GC tasks for a specific cluster. For example:

# Enable load-balancer and security-group tasks, target-group will be disabled.
clusterawsadm gc configure --cluster-name=cluster --gc-task load-balancer --gc-task security-group

# Reset configuration (remove the annotation)
clusterawsadm gc configure --cluster-name=cluster

Environment:

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

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