feat(http): offer more control over server sent events format

[thoresuenert]

The current implementation of the GenericResponseSender hardcoded the server sent event protocoll to:

 echo "event: {$event}\n";
 echo "data: {$data}";
 echo "\n\n";

We cannot use multiple data lines or add something like id or retry to the fields list https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events#fields.

I introduced a Interface ServerSentEvent with a hooked property $datalines to move the shape of the data transmitted with a single message into the Event itself.

This change allows us to implement the datastar protocoll for example:

event: datastar-patch-elements
data: elements <div id="hal">
data: elements     I’m sorry, Dave. I’m afraid I can’t do that.
data: elements </div>

[innocenzi]

Thank you!