feat(providerconfig): add namespaced v1beta2 credentials surface

[markussiebert]

Summary

Adds gitlab.m.crossplane.io/v1beta2 ProviderConfig as the storage version of
the namespaced provider config, with a credentials surface that fits the
namespaced scope:

• secretRef is a LocalSecretKeySelector, always resolved in the
  ProviderConfig's own namespace (no namespace field);
• the filesystem and environment sources are not offered — they reference
  provider-global state (the provider pod's filesystem / process environment)
  and cannot be scoped to a single namespace without breaking isolation;
• the credentials source enum is narrowed to None;Secret.

v1beta1 is retained (served, non-storage) and only gains a version-level
deprecation warning steering users to v1beta2. ClusterProviderConfig and
ProviderConfigUsage are unchanged (v1beta1 only) — they legitimately need a
namespaced secret selector / have no successor.

The provider resolves namespaced credentials via the v1beta2 ProviderConfig and
records the resolved namespace in Config.CredentialsSecretRef, so self-managed
token detection keeps comparing against the managed resource namespace.

Conversion

Strategy None (no webhook). The v1beta1 -> v1beta2 delta is only
dropped/optional fields and a narrowed enum, none of which needs data
transformation. Because v1beta2 is storage, fields it does not define
(secretRef.namespace, fs, env) are pruned when a v1beta1 object is written,
and the provider only ever reads v1beta2 — so namespaced credentials can no
longer be sourced across namespaces regardless of the version applied.

Backward compatibility

• v1beta1 schema is unchanged except the deprecation marker; existing v1beta1
  manifests and stored objects keep working.
• ClusterProviderConfig / ProviderConfigUsage CRDs are byte-identical to
  before.
• No functional regression versus current master: secretRef.namespace was
  already ignored (own-namespace enforced) and fs/env were never
  implemented. The only visible change is that these already-ignored fields no
  longer persist on v1beta1 (pruned at v1beta2 storage), plus the deprecation
  warning.
• Dropping v1beta1 in the future will require a storage-version migration; it is
  intentionally kept served here so nothing must migrate now.

Testing

• go build ./..., go vet ./..., go test ./... pass; make generate
  produces no drift.
• Live end-to-end against a real GitLab instance via a kwok cluster with the
  provider running out-of-cluster:
  • native v1beta2 owner-mode token create -> Ready;
  • v1beta2 self-managed detection (SelfManaged=True) and one forced
    self-rotation (token id changes, then settles, no rotation storm);
  • v1beta1 manifest -> None conversion -> Ready (deprecated namespace pruned
    at storage);
  • the v1beta1 deprecation warning is emitted on kubectl apply.

[henrysachs]

Thanks @markussiebert — this addressed everything from the last pass, and in a couple of places more cleanly than I suggested.

LGTM