ResponsableFactory resolves the transformer class by convention from the
model's namespace, so list/show calls on the new backup.agent resources were
throwing SYSTEM-EXCEPTION Target class [...Transformer] does not exist. Adds
the three Transformer + AbstractTransformer pairs, matching the
Buckets/Servers pattern. BackupAgentsTransformer deliberately omits
agent_api_key/registration_token from the response — unlike ServersTransformer
(admin-only), s3_backup_agents is customer-readable, so echoing the live NATS
credential back on every GET would leak it; those fields are only ever
returned once, from BackupAgentsService::register().