Allow schema propagation to client apps

[iboB]

Currently client apps have no way of accessing a provider schema unless they use the plib. This limitation can be easily lifted if the plugin is made available through make_ac_local_plugin_available, but non-local apps might also need the schema.

The current idea to propagate schemas independent of plugins was to have them in separate libraries, but this puts too much strain on development.

We should create a better plugin-independent way to propagate schemas.