This change adds two optional callbacks to both `DialOptions` and
`AcceptOptions`. These callbacks are invoked synchronously when a ping
or pong frame is received, allowing advanced users to log or inspect
payloads for metrics or debugging. If the callback needs to perform more
complex work or reuse the payload outside the callback, it is
recommended to perform processing in a separate goroutine.

The boolean return value of `OnPingReceived` is used to determine if the
subsequent pong frame should be sent. If `false` is returned, the pong
frame is not sent.

Fixes #246