Add support for contributions of export rules and checks to the language

[rmitin]

Sometimes certain class of sources of one given DSL needs to get more support: it infers more types, it has specific constrains, it exports those inferred types to the index. These extensions cannot and should not be part of the core plugin for the language. To implement them separately a mechanism for extensions is needed.

We already have a way to contribute type model inference extensions with com.avaloq.tools.ddk.xtext.modelinference.IAdditionalModelInferrer. Now we generalize it for export rules and checks.