Simple imoplementation done. Doing it the right way is in Okapi-238, so closing this one.

The stuff about system interfaces has got its own issue, Okapi-238. Continue the discussion there.

Later, we will have problems in accessing the interfaces Okapi does not know about (like admin of RMB modules). There we will have to be careful about many things, for example permissions, and of the admin function applies to any running instance of the module, any one instance, or all instances...

And I think we can implement this today, if we keep the tenantInterface field as a temporary, deprecated, way of specifying the same thing. That way, both forms of ModuleDescriptors would work. Later, when we make other breaking changes, we can drop the tenantInterface.

Hmm, the discussion on Slack seems to like the idea of these specially defined 'provides' interfaces. Mostly because there will be more of them. We have talked about a health check, the RMB already defines some kind of admin interface for setting a log level, and who knows what else, as well as some way to get database statistics...