Add ConnState hook for connection lifecycle monitoring

[mariuso]

This PR adds connection lifecycle hooks to enable monitoring, metrics, and debugging of SMTP connections.

What's new:

• ConnState callback field on Server that gets called when connections change state
• Connection states: New, Active, Auth, Data, StartTLS, Reset, Idle, Error, Closed
• Each state transition triggers the callback with the connection and new state

Use cases:

• Track active connections count
• Monitor connection duration and state transitions
• Debug authentication issues and connection problems
• Build metrics dashboards for SMTP server health

Example:

server.ConnState = func(conn net.Conn, state smtp.ConnState) {
    switch state {
    case smtp.StateNew:
        activeConnections.Inc()
    case smtp.StateClosed:
        activeConnections.Dec()
    }
}

No breaking changes - the callback is optional and everything works exactly the same if you don't set it.

Includes comprehensive tests showing all state transitions.

[arp242]

I think c.setState(StateError) can be in the Conn.WriteResponse()? That is:

func (c *Conn) writeResponse(code int, enhCode EnhancedCode, text ...string) {
	if code >= 400 {
		c.setState(StateError)
	}

	[..]
}

There's a bunch of error replies where you're not calling setState(stateError) though, I'm not sure if that's intentional?

[arp242]

Doesn't compile because conn.hasReceivedData doesn't exist; seems to be a left-over of your other PR (#285)?