Add RestAPI stage variables support

Since I started working on #1467 I occasionally run into something since those authorizers interact with so many things.

I wanted to write tests for authorization with stage variables, but I haven't found any means of setting those, so I think this is a missing feature.

My proposition is to get those through `config.json` with the key `stage_variables` that would behave the same as `environment_variables` (could be global and per stage) and user would be able to define his variables there.

From what I see the websockets have a bit different method for putting stage variables in so it could be a different feature-request

Start by tracing how config.json handles environment_variables, including global and per-stage values. Define the proposed stage_variables behavior for REST API authorization tests, and verify the scope boundary with websockets; done means users can set the values globally or per stage and tests can use them.

Written by the indexing model from the issue text.