API: Revise Connectors: OpenML

Connectors currently only run at the root node and connect directly to the database.
They also fail to update existing assets, and are stateful which is error prone.
They run in background processes, which is against docker best practices, and also have insufficient logging to perform posthoc diagnostics.

Ideally we just provide a more streamlined connector development experience, that's also based around REST API access instead of direct database access. This will make it easier for people to develop their own connectors, and may also simplify maintenance in the future.