Refactor proxy request authentication

[C0rby]

In #4374 the request authentication was refactored but this solution is still not very clean.
Before that PR the authentication logic had many implicit behaviors e.g. unauthenticated requests were still forwarded and reva has it's own authentication handler which is why this worked.

But the clean way would be to explicitly handle the authentication in the proxy service and only forward the requests when they were successfully authenticated or when the request is made to "unprotected" paths.

One way to do that would be to add an "unprotected" flag to the proxy routes. Then we would need to route the request before invoking the authentication middleware so that we get the extra information into the context.