(56) handle "scary config"

[ibihim]

What

Make it less likely, that rewrite params are used. They should be only used if necessary, like in the context of prom-label-proxy.

Why

• Consuming header and query params, but leaving them as is, is usually a great first step into a CVE.
• Letting the client modify authorization attributes is even worse.
• Might be a hard requirement for Prometheus use cases with prom-label-proxy.

[ibihim]

By @stlaz

Per our offline discussion, we should get rid of the query rewrites and stick with the header handling.

I'd suggest the switch for the handling logic to operate like a mode switch rather than the bool. We would have two modes:

1. TerminatingEvaluation
2. PassthroughEvaluation
or whatever names seem fitting, and document them properly, including the dangers of passthrough.

#346 (comment)

[ibihim]

I wanted to make a step back to the discussion phase, this is why I closed the previous PR.