Allow users to create a New mDNS Discovery service with context as an argument to make it cancellable

[phillipahereza]

Currently, the NewMdnsService function creates a new context, meaning it cannot be cancelled from the calling function.

To maintain compatibility, I suggest adding a new function, NewMdnsServiceWithContext(ctx context.Context, host host.Host, serviceName string, notifee Notifee), that accepts a context as an argument.

[sukunrt]

You can use the Close method to close the service. Explained here too: #3089 (comment)