[GEP-28] Support managing machines without target cluster for `gardenadm bootstrap`

[timebertt]

/kind enhancement

What this PR does / why we need it:

This PR implements changes to support provisioning control plane machines of autonomous shoot clusters in gardenadm bootstrap (GEP-28 medium-touch scenario). If --target-kubeconfig=none is configured, MCM skips all interaction with the target cluster as described in the referenced issue.

Which issue(s) this PR fixes: Fixes https://github.com/gardener/machine-controller-manager/issues/994

Special notes for your reviewer:

I built ghcr.io/timebertt/machine-controller-manager:gardenadm-bootstrap for testing this PR in a gardener local setup. You can check out (https://github.com/gardener/gardener/pull/12307) to test this PR with gardenadm bootstrap.

Release note:

machine-controller-manager can manage machines without a target cluster by specifying `--target-kubeconfig=none`. See the [documentation](https://github.com/gardener/machine-controller-manager/blob/master/docs/FAQ.md#how-to-run-mcm-in-different-cluster-setups) for more details.

[gardener-robot-ci-1]

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[timebertt]

@elankath @unmarshall thanks for your reviews. PTAL :)

[timebertt]

@elankath @unmarshall can you kindly take another look at this PR? :)