remote: Implement avahi support in HTTP endpoints

HTTP endpoints may announce themselves through DNS-SD. The _sparql._tcp service name is reserved for this kind of services:

http://dns-sd.org/ServiceTypes.html https://www.floop.org.uk/eagle/discovering-sparql

Implement Avahi support for our HTTP endpoints, so that these are properly discoverable.

Also, implement the CLI support to list HTTP endpoints, and the bash-completion support to tab-complete them too.