✨ Add support for autoscaling to/from 0

[nikParasyr]

What this PR does / why we need it:
Add support for autoscaling to/from 0. To do this:

• Follow the contract for doing this as documented here
• Add a new controller to patch OpenStackMachineTemplate.Status accordingly
• Add the feature behind a feature flag

Which issue(s) this PR fixes:
Fixes #1328

Special notes for your reviewer:
I'm currently not adding support for:

1. Architecture info: Could not find a standardized flavor extra_spec for this one. There are some traits/resource specific ones but they seem to be for when ironic is used. Images can have an architecture attribute but im not sure whether this should be used => I'd prefer to leave this out of this PR and see if users need/require this.
2. GPU info: Again couldn't find the standardized extra_spec for this. I would also need to check what type of info autoscaler expects ( just vendor info or even gpu model info etc ) => I'd prefer to leave this out of this PR and see if users need/require this.

TODOs:

• squashed commits
• if necessary:
  • includes documentation
  • adds unit tests
  • adds e2e tests

/hold

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[k8s-ci-robot]

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[k8s-ci-robot]

Hi @nikParasyr. Thanks for your PR.

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