client: hide alternate_name from API

client: hide alternate_name from API

The dentry alternate_name was exposed primarily to facilitate testing. Instead,
put these methods in the TestClient scaffolding to allow reading/manipulating
alternate_name.

Because we will be using alternate_name to handle encrypted names and the
normalized / case folded name, we do not want the application to change the
meaning of the metadata out-of-band.

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>
Fixes: https://tracker.ceph.com/issues/66373
(cherry picked from commit 1e34963af11210ebd940e20219728b91031e720f)