Support autoscaling from zero

[com6056]

/kind feature

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

Add support for autoscaling from zero defined in https://github.com/kubernetes-sigs/cluster-api/blob/main/docs/proposals/20210310-opt-in-autoscaling-from-zero.md

Just implemented upstream in kubernetes/autoscaler#4840

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):

[Skarlso]

/triage accepted

I saw this and wanted to do it! :)

[Skarlso]

/assign Skarlso

[Skarlso]

Ping @richardcase, @sedefsavas and @Ankitasw for a second opinion. :)

[Skarlso]

Also, now I'm wondering, when and how do we usually do triage-es?

[Skarlso]

My understanding from reading this is as follows:

• Create a Status field in AWSMachineTemplate and reconcile it
• Based on those templates, create annotations on the machine deployment following the names outlined within
• ?
• Profit.

I'm not sure what else is required. Do we just have to add the annotations based on our spec fields and reconcile them back into the status field? Is that it? :) ( ignoring validations and the likes for now ).

Maybe reconciling it back into the spec if the annotations exist, might disrupt some flow. Like, it would cause a diff since local resources don't contain those settings.

Found in the Code:

	// The annotations should override any values from the status block of the machine template.
	// We loop through the status block capacity first, then overwrite any values with the
	// annotation capacities.

So status IS something the autoscaler will pick up.

[Skarlso]

And preventing CAPA itself from creating a machine instance where it expects one to be if these fields are defined.

[Skarlso]

Aha

	infraref, found, err := unstructured.NestedStringMap(r.unstructured.Object, "spec", "template", "spec", "infrastructureRef")

[Skarlso]

Answer from cluster-api channel:

Yes, it's enough to define the status.
Reference implementation can be found here.