Add hook for providing project id in the header of every request

[rgrinberg]

The project id can now be added by initing the client module with an appropriate
provider that fetches the current project id. For example, from the client state.

[rgrinberg]

@AnthonyMl @bdionne

OK I almost made this prototype work. I'm not really convinced this is the right idea though so I really would like to talk about this before proceeding further.

The only thing that remains here is stripping off the project Id's before passing them to protege but all this really shows how fragile this entire approach is.