www/caddy-custom: Add port

This port allows you to build a version of the caddy webserver
using whichever plugins you desire. To do so, it uses the
official build mechanism, xcaddy (from the www/xcaddy port).

However, this requires network access.

To use this port, you'll need to add
ALLOW_NETWORKING_PACKAGES="caddy-custom" to your poudriere.conf.
Make really really REALLY sure you understand the implications
of doing this. If you use xcaddy, then there really isn't any
other actual implication, but I'm using my scary voice to
discourage people from shooting themselves in the foot.