Add ClusterClass support machine pools

[richardcase]

/kind feature
/priority important-soon
/triage accepted

Describe the solution you'd like

Now the MachinePool support for ClusterClass has merged upstream in CAPI we can add it to CAPA.

There isn't a huge number of docs on this. It's mainly creating new API types:

• AWSMachinePoolTemplate
• AWSManagedMachinePoolTemplate - but there is no point in doing this until AWSManagedCluster and AWSManagedControlPlane have templates (see Add AWSManagedControlPlaneTemplate type #3166)

Anything else you would like to add:

Some prior art:

• ✨ Add MachinePool workers support in ClusterClass cluster-api#9016
• Add ClusterClass Support for Managed Clusters cluster-api-provider-azure#4155

Environment:

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

[richardcase]

/assign
/lifecycle active

[richardcase]

Will do this at the same time as #3166

[k8s-triage-robot]

This issue is labeled with priority/important-soon but has not been updated in over 90 days, and should be re-triaged.
Important-soon issues must be staffed and worked on either currently, or very soon, ideally in time for the next release.

You can:

• Confirm that this issue is still relevant with /triage accepted (org members only)
• Deprioritize it with /priority important-longterm or /priority backlog
• Close this issue with /close

For more details on the triage process, see https://www.kubernetes.dev/docs/guide/issue-triage/

/remove-triage accepted

[richardcase]

/triage accepted