rgw: implement GetObjectAttributes

rgw: implement GetObjectAttributes

Implements the corresponding S3 operation, and
introduces a new Object::list_parts SAL interface to support it.

Includes Casey Bodley <cbodley@redhat.com>:
  * use uncompressed part size
  * local variable shadowed a member variable and broke handling of
      PartNumberMarker in request and response

Fixes: https://tracker.ceph.com/issues/64109
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>