Add a generic "call" operation

CMIS never indented to cover all features and capabilities of a repository. But sometimes a repository wants to expose a few features that are not part of the CMIS spec and probably never will.
A few examples: basic recycle bin operations (list, restore, permanently delete), publishing a document, triggering the generation of a rendition.

It would be nice to a have generic "call" operation that takes an undefined input (stream of bytes) and returns an undefined output (stream of bytes).
Most CMIS clients would ignore it but if a client knows the repository it can easily call specific operations without leaving the CMIS realm (endpoint, binding, session, authentication,etc.).

I'm not advocating that repository vendors should expose each and every repository feature through this. That's what proprietary APIs are for. I'm just talking about a few, selected operations that makes sense in the CMIS context and play nice with the CMIS data model.