Add NetworkConfig.AllowedDomains and DeniedDomains fields for controlling
outbound connections by hostname. Deny rules are checked first (deny wins).
When AllowedDomains is set, only matching domains are permitted. When only
DeniedDomains is set, all domains except denied ones are allowed.
Implement FilteringProxy that wraps gost HTTP proxy with domain enforcement
via AllowConnect callback. Skip GreyHaven proxy/DNS defaults
