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/config/config.go

@@ -28,6 +28,7 @@ type Config struct {
 	ProxyProtocolListener    TypeBool        `json:"proxyProtocolListener"`
 	PreferIP                 TypePreferIP    `json:"preferIp"`
 	DomainFrontingPort       TypePort        `json:"domainFrontingPort"`
+	DomainFrontingIP         TypeIP          `json:"domainFrontingIp"`
 	TolerateTimeSkewness     TypeDuration    `json:"tolerateTimeSkewness"`
 	Concurrency              TypeConcurrency `json:"concurrency"`
 	Defense                  struct {