Add AUTH keys as specified in proposal 176

Add AUTH keys as specified in proposal 176

Our keys and x.509 certs are proliferating here.  Previously we had:
   An ID cert (using the main ID key), self-signed
   A link cert (using a shorter-term link key), signed by the ID key

Once proposal 176 and 179 are done, we will also have:
   Optionally, a presentation cert (using the link key),
       signed by whomever.
   An authentication cert (using a shorter-term ID key), signed by
       the ID key.

These new keys are managed as part of the tls context infrastructure,
since you want to rotate them under exactly the same circumstances,
and since they need X509 certificates.