Change the way user credentials for authz are provided to not be in context

Description

Currently, the credentials to authenticate/authorize are send in the context. This works well for go but not if the communication via gRPC/REST is done from an application written in another language. These apps would not be able to get authorized to do anything in any way. Therefore, the credentials should be provided in another way. Ideas welcome!

This change will introduce lots of changes in other areas, to be more precise, possibly for our own applications, the tests, the gRPC API abstraction thing and gosdnc. So, implementing a new way of providing the credentials will lead to many small adjustments. Might take some time to find them all.