Add endpoint with list of OCS providers

This adds a OCS provider list at `ocs-provider/` to enable a somewhat autodiscovery of services.

In the moment it is not possible for applications to add own entries and it is basically hard-coded in the controller. I'm aware that this is not an optimal solution and I'm happy for any pragmatic input that won't pollute our public API too much. (and is not a big super hack, this is already hacky enough ;-))

That said, I'd consider it as a first step also feasible to go with this somewhat hard-coded list and see if this gets adopted...