feature request: let "TXT" can be change dynamically

## Description:
According to [RFC 6763](https://datatracker.ietf.org/doc/html/rfc6763#page-11), **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:
```golang
service, _ := mdns.NewMDNSService(host, "_foobar._tcp", "", "", 8000, nil, info)
// this is expectation
service.UpdateTxt(…)

```

Start by tracing mdns.NewMDNSService and how the initial info/TXT values are handled. Determine how a service can expose the requested UpdateTxt operation after creation and how changed device status is advertised. Done means callers can dynamically update the TXT description for an existing service.

Written by the indexing model from the issue text.