Add domain-fronting-ip option

Allow specifying an explicit IP address for the domain fronting host
instead of relying on DNS resolution. Useful when DNS resolution of
the fronting hostname is blocked.

The hostname from the secret is still used for SNI in TLS handshake.

internal/cli/run_proxy.go

@@ -260,6 +260,7 @@ func runProxy(conf *config.Config, version string) error { //nolint: funlen
 
 		Secret:             conf.Secret,
 		DomainFrontingPort: conf.DomainFrontingPort.Get(mtglib.DefaultDomainFrontingPort),
+		DomainFrontingIP:   conf.DomainFrontingIP.String(),
 		PreferIP:           conf.PreferIP.Get(mtglib.DefaultPreferIP),
 
 		AllowFallbackOnUnknownDC: conf.AllowFallbackOnUnknownDC.Get(false),