Draft: dns over https

Current WiP for DoH, on top of 1840-netmgr-tls-layer 'Big' things missing:

1. Tests
2. dig integration - dig should call isc_nm_doh_request()
3. Listening - parsing the http server statement into isc_nm_listenhttp2 call
4. Endpoint - paring the http endpoint statement into isc_nm_http_add_entpoint
5. Reconfig -endpoint/listener removal
6. Paring of 'GET' arguments (currently only POST requests are supported)