Initial import of DNS over CoAP

[miri64]

This is based on the work by Philipp Herold (@powerbuilder1).

I cleaned up most of the code during the IETF 122 hackathon and the IETF week. There are definitely some things that still need cleaning (see checklist below) but I wanted to open a PR before the end of IETF week (and before my vacation until the 31st) to gather your opinions.

Things still to do:

• Make OSCORE credentials non-constant and configurable
• Make CoAP resource path non-constant and configurable
• Find out why libcoap sends a piggybacked ACK-message for late responses instead of the correct CON-message
• Reuse TLS-PKI for DTLS

[miri64]

Rebased to current master to resolve merge conflict.

[miri64]

2/4 TODOs done. I see if I can manage the rest during the hackathon or if this is something I leave over for Montreal (or before if I find the time during the last bouts of my PhD).