feature request: let "TXT" can be change dynamically

[XiXiangFiles]

Description:

According to RFC 6763, TXT is a description of this service that supports features and more detail. The descriptions of the devices have demands for dynamic changes in device status change.
Example:

service, _ := mdns.NewMDNSService(host, "_foobar._tcp", "", "", 8000, nil, info)
// this is expectation
service.UpdateTxt(…)