Add Draft "Choice of identity" Decision Record

Created by: joshmue

While having not worked extensively with any of the mentioned options yet (and possibly misunderstanding some details), I tried to give a brief assessment of possible options for machine (and human) identities which could be used in a generic manner to authenticate against cloud services.

This document must not be merged before #140 is accepted as SCS consensus decision. Until then, its purpose is to share my thoughts about this topic, enabling future discussions.