Enable tags query over the query socket

This patch adds the tags field to the objects which were missing it
(all except Cluster), implements handling the LuxiCall QueryTags, and
then enables the use of the query socket in cli.ListTags, used by all
commands, and in the RAPI client.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>