Cluster-api-provider-aws: Implement AWSMachinePools using ASG

[CecileRobertMichon]

/kind feature

Describe the solution you'd like
[A clear and concise description of what you want to happen.]

Implement AWSMachinePool using Auto Scaling Group. The AWSMachinePool implementation should follow the same package / group structure as CAPI and be added to the exp package, the experimental feature package. The feature should be gated using the MachinePool flag used in CAPI.

See also: https://github.com/kubernetes-sigs/cluster-api-provider-azure/pull/483 and CAPI Machine Pool Proposal

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):

[detiber]

/priority important-longterm

[detiber]

/help

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[gab-satchi]

/assign

[krmayankk]

@gab-satchi are you working on this already or plan to work on it ? when can we get this feature ?

[rudoi]

We're actually working on this currently as part of adding EKS Support (it should still work with the Kubeadm provider).

[randomvariable]

/assign @mytunguyen
/lifecycle active

[randomvariable]

Fixed in #1860