Add Draft "Choice of identity" Decision Record

[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.

[mbuechse]

Is this still relevant? I will close this PR if nothing happens by July 31st.

[joshmue]

As commented in #140, it's still generally relevant.

But: The decision what type of identity (document) should be used, may be more tilted towards OIDC style tokens, currently.

[fkr]

same as #140 - closed, since nobody is taking care of this.