feat(masque): make TLS SNI configurable via server_name

MASQUE outbound previously hardcoded the TLS SNI to consumer-masque.cloudflareclient.com. Add a server_name field to the MASQUE outbound TLS options. When empty it falls back to the existing default (cloudflare.ConnectSNI), so existing configs are unaffected.

option/masque.go

@@ -22,6 +22,7 @@ type MASQUEOutboundOptions struct {
 }
 
 type MASQUEOutboundTLSOptions struct {
+	ServerName            string                              `json:"server_name,omitempty"`
 	Insecure              bool                                `json:"insecure,omitempty"`
 	CipherSuites          badoption.Listable[string]          `json:"cipher_suites,omitempty"`
 	CurvePreferences      badoption.Listable[CurvePreference] `json:"curve_preferences,omitempty"`