Issue description
A customer wants to send bulk measurements via thin-edge, each measurement has different timestamps. The current thin-edge JSON format can include timestamp only on the top level. But C8Y JSON supports createBulk.

Then I found that thin-edge dosen't support createBulk topics out-of-the-box.

Workaround
User can still use createBulk topics by:

1. Add the topic into c8y-bridge.conf, for example: topic measurement/measurements/createBulk out 2 c8y/ ""
2. Restart mosquitto service: systemctl restart mosquitto.service
   Then thin-edge can forward the measurements to the platform. But thin-edge will lose this custom config after reconnecting tedge.

Expecting solution
It would be great if thin-edge can support createBulk topics natively. According to C8Y documentation, C8Y supports:
event/events/createBulk
alarm/alarms/createBulk
measurement/measurements/createBulk