Moving from the POC in this PR we need to create a proper infrastructure for the plugin. The infrastructure should include just:

• registering method for the plugins
• one sample API (e.g. upgradeMetadata/downgradeMetadata) with the required machinery to call the corresponding functions in the plugins
• interface to be implemented for the plugins (with empty defaults, so that a plugin can just implement what it needs)
• a proper directory structure for the plugins

We want to keep the skeleton simple, and add the infrastructure functionality as part of the other tickets rather than loading this ticket with all of them. This will help keep the infrastructure light, at the cost of possibly having to redo some of the work.