Add custom ping message option to Ping function

```
func (c *Conn) Ping(ctx context.Context) error {
p := atomic.AddInt32(&c.pingCounter, 1)

err := c.ping(ctx, strconv.Itoa(int(p)))
if err != nil {
return fmt.Errorf("failed to ping: %w", err)
}
return nil
}

func (c *Conn) ping(ctx context.Context, p string) error {
```

Is it possible to add a parameter to use custom ping/pong message when sending it? Sometimes, one side want to listen to specific ping/pong message. If we can add that, it would be very helpful.
I really like this project. It is simple enough that I can just focus my project logic.

Start by locating Conn.Ping and the internal c.ping function shown in the issue, then inspect how the current ping payload is generated and covered by the surrounding tests. Define the custom-message API and update the relevant behavior so callers can send the requested ping/pong message while preserving the existing default behavior.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.