Harmonize Applier, ChartRenderer and ChartApplier creation

[timebertt]

How to categorize this PR?

/area quality
/kind enhancement
/priority normal

What this PR does / why we need it:
This PR harmonizes/centralizes how Applier, ChartRenderer and ChartApplier instances are obtained in g/g.
It removes any seperate constructions of ChartRenderers and ChartAppliers in favor of two respective functions that were added to pkg/client/kubernetes.Interface to obtain such instances.
This is needed to ensure, that we use the same controller-runtime clients while applying Charts everywhere to be able to leverage the clients' caches (which is not possible, if we construct new clients, ChartAppliers and so on, wherever needed).

Which issue(s) this PR fixes:
Part of #2414

Special notes for your reviewer:
This PR also contains one commit from #2415, as it depends on it.
I opened this as a draft PR to collect early feedback and will rebase and drop the commit once #2415 has been merged.
/invite @gardener/gardener-maintainers

✅ Depends on #2415

Release note:

The `pkg/client/kubernetes.ApplierInterface` interface has been renamed to `Applier`. Please adapt your usages accordingly when revendoring.

You can now obtain a `ChartRenderer` and a `ChartApplier` by using the respective functions of the ClientSet interface (`pkg/client/kubernetes.Interface`). Please use the provided functions instead of constructing new `ChartRenderer`s and `ChartApplier`s wherever needed.

[timebertt]

removed commit from #2415

/ready-for-review
/needs review

[rfranzke]

Very nice PR... no comments from my side, other than:
/lgtm

[plkokanov]

Just some minor stuff. Otherwise looks good.

[plkokanov]

/lgtm

[gardener-robot]

@plkokanov Command /lgtm is not available to you but only to a Maintainer.