Make drain package reusable for controller-runtime based callers

[acumino]

/kind enhancement

What this PR does / why we need it:
This PR extends the drain Options API to support callers (e.g. Gardener) that do not run client-go informers and need to plug in their own pod source or customize pod selection.

• Add PodProvider interface (PodsForNode) and SetPodProvider setter — allows callers to supply pods via a controller-runtime client instead of a pod lister/informer.
• Add AdditionalPodFilters field — caller-supplied predicates evaluated alongside the built-in drain filters.
• Add SkipVolumeHandling flag — skips PV detach/reattach during eviction, independent of ForceDeletePods.
• Make podSynced cache-sync conditional — skipped when no informer is running (podSynced == nil).
• Make pdbLister nil-safe so callers that don't set one don't panic.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

The drain package now exposes a `PodProvider` interface, `AdditionalPodFilters`, `SkipVolumeHandling`, and a nil-safe `pdbLister` guard, allowing external controllers (e.g. Gardener) to reuse the drain logic without running a client-go informer stack.

[gardener-prow]

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[acumino]

/kind enhancement

[takoverflow]

/assign