chore: one off SMS campaign APIs (#2589)

app/views/api/v1/models/_campaign.json.jbuilder

@@ -9,7 +9,13 @@ json.sender do
   json.partial! 'api/v1/models/agent.json.jbuilder', resource: resource.sender if resource.sender.present?
 end
 json.message resource.message
+json.campaign_status resource.campaign_status
 json.enabled resource.enabled
+json.campaign_type resource.campaign_type
+if resource.campaign_type == 'one_off'
+  json.scheduled_at resource.scheduled_at
+  json.audience resource.audience
+end
 json.trigger_rules resource.trigger_rules
 json.created_at resource.created_at
 json.updated_at resource.updated_at