onSendHookHandler: no option for payload type

When implementing a function for onSend hook - there is no option to pass a type for onSendHookHandler and it's assuming that the payload is of `unknown` type

```js
// this code is not working:
export const myHook = (): onSendHookHandler => {
return async (req: FastifyRequest, rep: FastifyReply, payload: string) => {
....
}
}

// this code is working:
export const myHook = (): onSendHookHandler => {
return async (req: FastifyRequest, rep: FastifyReply, payload: unknown) => {
....
}
}
````

If onSendHookHandler supports generics - why we can't use it when declaring onSend:
```
onSend?: onSendHookHandler | onSendHookHandler[];
```

## Your Environment

- *node version*: 14
- *fastify version*: >=3.22.0
- *os*: Macx