Support stack config as a source for required path params

[praneetloke]

For example, in Scaleway's APIs, the zone path param is required in nearly all API endpoints. It's redundant to have to set it in every resource. It would be nice if the value was sourceable from stack config. We could provide a callback interface that would allow implementations an opportunity to supply the required path params before the HTTP request is created. That info can be merged with the regular search for path params that this framework already does. project is another example.