The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behavior

Currently middlewares are limited to hook before the server aknowledges a clients message, but it can't hook on the client acknowledging from the client.
Currently data validation logic needs to be rewritten in the logic of every emited with acknowledgements, that may result in a lot of boiler plate code.

New behavior

Introduced the useOutgoing((event: [string, ...any[]], next: (err?: Error) => void) => void) method.
It is behaving much like regular use() does, but before the messages are being sent from the server to the client.
(They do run before the catch all listeners and before the acknowledgement callback gets popped to the ack manager in order to be able to edit the event, or abort it).

Example:

io.on("connection", (socket) => {
 socket.useOutgoing((event, next) => {
 if (event.length && typeof event[event.length - 1] === 'function') {
 const callback = event[event.length - 1];
 event[event.length - 1] = (...args) => {
 // able to hook right before the acknowledgement callback is executed.
 callback(...args);
 }
 }
 // do not forget to call next
 next();
 });
});

Other information (e.g. related issues)

I disscussed other option briefly in this issue #4882.
For short, the tricky part about this, is that one might want access to the context of the event that originated the ack repsonse whilst consuming what the client acknowledged. Else it would be of no use hooking there, except for logging purposes I guess.
This would be a solution that doesn't disrupt current usage of Socket.IO, and we have access to the context of the event.