refactor(kms): consolidate encryption metadata key constants into their shared home (#5995)

The shared module rustfs_utils::http::object_encryption_keys is the single source of truth for encryption metadata key names, but three call sites still carried their own copies or bare literals: crates/kms/src/service.rs (two private constants plus four bare x-rustfs-encryption-* literals on both the write and read path), rustfs/src/app/select_object.rs (six SELECT_* copies), and rustfs/src/storage/options.rs (two private prefix copies now imported from header_compat). All values are unchanged, so the change is a compiler-verified rename.

The reader-only x-rustfs-internal-server-side-encryption- family gets a named constant with the verified judgment recorded on it: no writer emits these keys anywhere in the repo (the SSE writer persists the MinIO-branded keys verbatim for interop), the two comments claiming the dual-key invariant writes this twin were wrong and are corrected, and the defensive redaction/strip readers are kept because removing them is risk-asymmetric.

rustfs-kms's rustfs-utils dependency now declares the http feature it uses instead of relying on feature unification from sibling crates.

Refs rustfs/backlog#1775, rustfs/backlog#1562.

crates/filemeta/src/fileinfo.rs

@@ -277,9 +277,10 @@ pub struct FileInfo {
 /// Values of these keys must never reach logs at any level.
 fn is_sensitive_metadata_key(key: &str) -> bool {
     // `is_encryption_metadata_key` covers the x-minio-internal- SSE prefix but not
-    // its x-rustfs-internal- twin, which the dual-key invariant writes alongside it.
+    // its reserved x-rustfs-internal- twin, which has no writer today but must
+    // stay redacted in case one appears.
     is_encryption_metadata_key(key)
-        || starts_with_ignore_ascii_case(key, "x-rustfs-internal-server-side-encryption-")
+        || starts_with_ignore_ascii_case(key, rustfs_utils::http::RUSTFS_INTERNAL_ENCRYPTION_PREFIX)
         || rustfs_utils::http::REPLICATION_SSE_TRANSPORT_PREFIXES
             .iter()
             .any(|prefix| starts_with_ignore_ascii_case(key, prefix))