feat(storage): move DecodeCallback to storage package, add SetFeatureSupported

[zachsmith1]

Summary

Cherry-picks 32f5e90 off the `feat/per-cluster-allocators` branch so the relocation can land independently of the larger per-cluster-allocators work. The change is a pure prerequisite for KPEP-0001 (pluggable storage backends) — without it, non-etcd backends cannot honor the cacher-installed decode callback.

Two related changes in one commit:

1. `DecodeCallback` / `WithDecodeCallback` / `DecodeCallbackFromContext` move from `k8s.io/apiserver/pkg/storage/etcd3` → `k8s.io/apiserver/pkg/storage`. Backwards-compat aliases stay in `etcd3` (marked `Deprecated:`) so any in-tree caller still compiles.
2. `feature.SetFeatureSupported()` added to the storage feature checker so non-etcd backends (Spanner) can declare native support for capabilities like `RequestWatchProgress` without bolting on a fake etcd implementation.

Why now

KPEP-0001 lands a registry of pluggable storage backends in `kplane-dev/storage`, with Spanner as the first non-etcd consumer. Spanner's source already uses `storage.WithDecodeCallback`, but no merged fork commit exposes it at the `storage` package — Spanner doesn't build standalone today. Pulling this commit out of `feat/per-cluster-allocators` (which has unrelated changes still in flight) unblocks:

• `kplane-dev/storage` PR — registry types
• `kplane-dev/spanner` PR — `Register()` wrapper
• `kplane-dev/apiserver` PR — dispatch swap

…all of which can pin against this branch via pseudo-version while we iterate, then move to `main` once this merges.

Files

• `staging/src/k8s.io/apiserver/pkg/storage/decode_callback.go` — new (42 lines)
• `staging/src/k8s.io/apiserver/pkg/storage/etcd3/decode_callback.go` — now a thin shim of deprecated aliases
• `staging/src/k8s.io/apiserver/pkg/storage/cacher/lister_watcher.go` — 3-line import update so the cacher uses the new location
• `staging/src/k8s.io/apiserver/pkg/storage/feature/feature_support_checker.go` — adds `SetFeatureSupported()`

Test plan

• `go build ./pkg/storage/... ./pkg/storage/etcd3/... ./pkg/storage/feature/...` clean.
• `kplane-dev/spanner` with go.mod fork pin bumped to this branch's HEAD: `go build ./...` clean (currently fails — that's the symptom).
• In-tree etcd3 callers via the deprecated aliases compile unchanged.

Original commit author/co-author preserved from the cherry-pick.

[zachsmith1]

Closing in favor of pinning directly against feat/per-cluster-allocators — that branch already contains this cherry-pick (32f5e90) plus the per-cluster allocator helpers the apiserver also needs. The KPEP-0001 PR chain (kplane-dev/storage#2, kplane-dev/spanner#1, kplane-dev/apiserver#TBD) will pin against feat/per-cluster-allocators HEAD instead of this branch, and move to whichever main SHA the per-cluster-allocators feature lands on.