Export mdnsService struct or rename the Service interface for mDNS

Nobody has claimed this yet.

I would like to have my own struct, where I can assign mDNS service to [`mdns.NewMdnsService()`](https://github.com/libp2p/go-libp2p/blob/578af0c651a61549cc51a08c70f611c8977ae75e/p2p/discovery/mdns/mdns.go#L53):

```go
type MyStruct struct {
mdnsService *mdns.mdnsService // Error: name mdnsService not exported by package typesinternal#UnexportedName
}

// Meaning, this is currently NOT possible
MyStruct.mdnsService = mdns.NewMdnsService(p2pHost, "something", &mDNSNotifee{host: p2pHost})
if err := MyStruct.mdnsService.Start(); err != nil {
panic(err)
} else {
log.Println("mDNS service started")
}
```

If you would expose the `mdns.mdnsService` struct (just like you already do expose `dht.IpfsDHT`), I could assign it to my own struct.

Meaning it be much easier to later call a `MyStruct.mdnsService.Close()` on it for example. Maybe its just me, but I think it makes sense to export a struct that is getting returned by a public function.

Start in p2p/discovery/mdns/mdns.go at NewMdnsService and inspect the returned service's lifecycle methods, especially Start and Close. Compare the requested exported struct with the existing public API and determine which naming or exposure approach preserves usable mDNS startup and shutdown behavior.

Written by the indexing model from the issue text.