Example of backend query migration

[andresmgot]

Example of how the migration handler can be used for a backend plugin.

Required:
grafana/grafana-plugin-sdk-go#1029

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}