Introduce a listener for JSON events

[sbordet]

The first use case is when a client sends a message and may want to limit the size of the message being sent.
This listener would need to be called after all transformations that extensions may do to the message.
Furthermore, the JSON generation must be done once, so that applications can interact with the JSON without the need to generate the JSON a second time which would waste resources.

[sbordet]

It may be interesting to emit an event for the incoming JSON (on both client and server) before it is converted to a message object.
However, this would require to accumulate the JSON into a String, while currently we can parse the JSON using a Reader with much less impact on memory.

On server side another event may be emitted when the message is frozen, so that applications will be able to e.g. log the message length in bytes.