feat: add support for Midjourney relay mode based on path prefix

relay/constant/relay_mode.go

@@ -84,6 +84,8 @@ func Path2RelayMode(path string) int {
 		relayMode = RelayModeRealtime
 	} else if strings.HasPrefix(path, "/v1beta/models") || strings.HasPrefix(path, "/v1/models") {
 		relayMode = RelayModeGemini
+	} else if strings.HasPrefix(path, "/mj") {
+		relayMode = Path2RelayModeMidjourney(path)
 	}
 	return relayMode
 }