Talk to radosgw directly via go-ceph + aws-sdk-go-v2

The operator previously drove the Ceph manager dashboard REST API to manage
RGW users, buckets and policies. That coupled it to a dashboard login, the
dashboard's RGW wiring, and the dashboard's bucket API surface. Rebuild the
Ceph integration to talk directly to radosgw the way the CLI does, using
native Go libraries, while keeping every operator capability identical.

The exported surface of internal/ceph is unchanged, so the three controllers
and cmd/operator are untouched (bar the env-var/config plumbing already in
flight for the radosgw move).

- replace the internal/ceph client internals with github.com/ceph/go-ceph
  rgw/admin (Admin Ops API) for users, keys, quotas and bucket info/removal
- add github.com/aws/aws-sdk-go-v2 S3 client for bucket create, versioning,
  policy, tagging and object lock, signed as the bucket owner
- map go-ceph admin.ErrNoSuch*/ErrUserExists and smithy APIError codes into
  IsNotFound/IsConflict so controller create-vs-update branching is preserved
- set S3 path-style addressing and WhenRequired checksum modes for RGW
- delete the hand-rolled SigV4 signer, canonical-query and XML marshaling
- keep policy.go/BuildBucketPolicy/BuildTagJSON as pure builders
- replace the SigV4 signer tests with NewClient validation and error-classifier
  tests
- keep CGO_ENABLED=0 distroless: only go-ceph's pure-Go rgw/admin is imported
- rewrite README and docs/ceph-setup.md for the single RGW admin user
  (caps users=*;buckets=*) and CEPH_RGW_* credential Secret

Claude-Session: https://claude.ai/code/session_016CEncETbf8cvy1PhsHfFHM

api/v1alpha1/objectstoreuser_types.go

@@ -5,7 +5,7 @@ import (
 )
 
 // ObjectStoreUserSpec defines a Ceph RGW (S3) user. The operator creates the
-// user through the Ceph dashboard API and writes its generated access/secret
+// user through the radosgw Admin Ops API and writes its generated access/secret
 // key pair into a Kubernetes Secret. The key material is never stored on the
 // resource itself.
 type ObjectStoreUserSpec struct {