mgr/dashboard: fix zone update API forcing STANDARD storage class

mgr/dashboard: fix zone update API forcing STANDARD storage class

The zone update REST API (`edit_zone`) always attempted to configure a
placement target for the `STANDARD` storage class, even when the request
was intended for a different storage class name.
This caused failures in deployments where `STANDARD` is not defined.

Changes:
Club add placement target and add storage class methods into one single
add_placement_targets_storage_class_zone method which takes the storage
class as a param as well alongside the rest of the placement params.

Fixes: https://tracker.ceph.com/issues/73105
Signed-off-by: Aashish Sharma <aasharma@redhat.com>