✨ Add AWSMachines to back the EC2 instances in AWSMachinePools and AWSManagedMachinePools

[AndiDog]

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What type of PR is this?

/kind feature

What this PR does / why we need it:

This implements the CAPI MachinePool Machines proposal.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes https://github.com/kubernetes-sigs/cluster-api-provider-aws/issues/4184

Special notes for your reviewer:

Revival of https://github.com/kubernetes-sigs/cluster-api-provider-aws/pull/4527. I took @cnmcavoy's great effort and got it fully working. I tested EC2 cluster creation, machine deletion by user, instance creation and termination by ASG scaling, instance termination by user, ...

Checklist:

• [x] squashed commits
• [ ] includes documentation
• [x] includes emojis
• [x] adds unit tests
• [ ] adds or updates e2e tests

Release note:

Add AWSMachines to back the EC2 instances in AWSMachinePools and AWSManagedMachinePools