Automate schema upgrades

[tokle]

In our CD environment we upgrade our Athenz instance regularly to the latest version. From time to time there are sql schema updates (mentioned in release notes) that breaks the upgrade process, and we need to manually resolve the issues by applying the necessary changes. It would be great if the server could apply the schema changes during startup.

This could be done with e.g. Flywaydb, but there are likely other alternatives.