[Feature] Ensures the selectors for deployments used by DWD abide to the Gardener Extension concept

[ashwani2k]

Feature (What you would like to be added):
With GEP-01 (extensibility) cloudprovider specific details are extracted to extensions.
With the introduction of scaleRef dependencies in the release v0.7.0. The component model of gardener/gardener is broken as it requires it to have knowledge of machine-controller-manager deployment name. Details can be checked here.

As currently its resolved by adding a constant in the g/g api types to identify MCMDeploymentName as design with an alternate selection criteria will require a re-design of concepts which the basic functionality of DWD relies on.

Motivation (Why is this needed?):
To ensure we don't break the contract and keep the separation of concern design principle in play.

Approach/Hint to the implement solution (optional):

[ialidzhikov]

Let's close for now as currently I don't know of a provider extension that does not use MCM currently. If there is one in the future, we could think of a approach similar the logging extensibility where provider extensions provide a ConfigMap (filters, parsers) with a well-known label and these ConfigMaps are aggregated and passed to the fluent-bit on Seed bootstrap. Let's for now do not introduce such complexity when there is no real need for it.

/close