The `onConnect` and `setNotificationHandler` callback should provide the `ServerSession`

Is your feature request related to a problem? Please describe.
I've been trying to create a reactive listener based on per-session roots notification and found it impossible or wired.

onConnect {
    sessions.forEach { (_, session) ->
        session.setNotificationHandler<RootsListChangedNotification>(Method.Defined.NotificationsRootsListChanged) {
            scope.launch {
                session.listRoots()

                // do Something with roots per session
            }
            CompletableDeferred()
        }
    }
}

I have to reset notifications for all sessions on each new session to properly get the per-session info.

Describe the solution you'd like

onConnect { session : ServerSession -> 
// do something with session
}

or

server.setNotificationHandler<RootsListChangedNotification>(Method.Defined.NotificationsRootsListChanged) {session : ServerSession -> 
// do something with session
}

or both.

Other infos

I suspect that the no-session-arg style onConnect and other callbacks are the legacy of the old new server per transport architecture.

Start by tracing the existing onConnect and setNotificationHandler callback entry points and compare them with the ServerSession examples in the issue. Determine which callback API should receive the session, then verify that per-session roots notifications can be handled without resetting handlers for all sessions.