Use constant TXT challenge value

[realaravinth]

Currently, forge instance ownership TXT challenge creates a random string for TXT value. We could use TXT records as a way to provide/deny access to forges.

TXT starchart-{{ starchart instance's hostname }}.{{ forge instance's hostname}} spidering={{ allowed | false }},rate={{ requests per minute }}

This way, the forge admin will be able to limit which starchart instances have what privileges