Request queue schema file specification [work in progress]

Currently, this is neither specified nor implemented. We think that request queue schema might be useful for two things:

ensuring what kind of URLs might be enqueued (certain domains or subdomains, ...)
ensure that for example each request has userData.label, i.e. schema of userData the same way as we enforce it for the Datasets

We should consider renaming RequestQueue to just Queue and make it more generic, and then it makes sense to have request schema.

This is to be done