Reconsider different auth scheme design

[dborovcanin]

ENHANCEMENT

1. Describe the enhancement you are requesting:
   I don't like having both token and key in the list request. I prefer that apiutil.ExtractAuthToken(r) can decide what's the auth scheme and that auth gRPC client knows how to authorize different tokens. This can be done by either separating APIs or updating auth gRPC. Anyways, this is not related to this PR, and we'll create a separate issue from this comment to address it later and not to block this PR.

Originally posted by @dusanb94 in https://github.com/mainflux/mainflux/pull/1568#r819431029

2. Indicate the importance of this enhancement to you (must-have, should-have, nice-to-have).
   This is a mus-have enhancement.