Unable to read response cookies until onSend

### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have searched existing issues to ensure the bug has not already been reported

### Fastify version

4.25.2

### Plugin version

9.2.0

### Node.js version

20.9.0

### Operating system

Linux

### Operating system version (i.e. 20.04, 11.3, 10)

20.04

### Description

Hello,

In my route handler, if I use `reply.setCookie` to set a response cookie, I'm unable to read it inside my handler by calling `reply.getHeader('set-cookie')`. However, I can read my resp cookie later in the lifecycle (onSend and onResponse).

I need to read it inside my handler. Maybe it's working as intended or I missed something ?

### Steps to Reproduce

Server :
````javascript
const fastify = require('fastify');
const fastifyCookie = require('@fastify/cookie');

async function testCookiePlugin() {
const server = fastify();
server.register(fastifyCookie);

server.addHook('preSerialization', async function (request, reply) {
console.log('--------- server preSerialization -------------');
console.log('Read response cookies through reply.getHeader(\'set-cookie\')', reply.getHeader('set-cookie'));
});

server.addHook('onSend', async function (request, reply) {
console.log('--------- server onSend -------------');
console.log('Read response cookies through reply.getHeader(\'set-cookie\')', reply.getHeader('set-cookie'));
});

server.addHook('onResponse', async function (request, reply) {
console.log('--------- server onResponse -------------');
console.log('Read response cookies through reply.getHeader(\'set-cookie\')', reply.getHeader('set-cookie'));
});

server.route({
method: 'GET',
url: '/',
handler: async function (request, reply) {
console.log('--> Set resp cookie through reply.setCookie');
reply.setCookie('through-plugin', 'cookie set through reply.setCookie');
console.log('Read response cookies through reply.getHeader(\'set-cookie\')', reply.getHeader('set-cookie'));

console.log('--> Set resp cookie through reply.header');
reply.header('set-cookie', 'through-header=cookie set through reply.header; Secure; HttpOnly');
console.log('Read response cookies through reply.getHeader(\'set-cookie\')', reply.getHeader('set-cookie'));

return {
setCookieHeader: reply.getHeader('set-cookie')
};
},
preSerialization: async function (request, reply) {
console.log('--------- route preSerialization -------------');
console.log('Read response cookies through reply.getHeader(\'set-cookie\')', reply.getHeader('set-cookie'));
},
onSend: async function (request, reply) {
console.log('--------- route onSend -------------');
console.log('Read response cookies through reply.getHeader(\'set-cookie\')', reply.getHeader('set-cookie'));
},
onResponse: async function (request, reply) {
console.log('--------- route onResponse -------------');
console.log('Read response cookies through reply.getHeader(\'set-cookie\')', reply.getHeader('set-cookie'));
}
});

return server.listen({port: 8080, host: '0.0.0.0'});
}

return testCookiePlugin();
````

The following curl `curl -i http://localhost:8080/` gives me this :
````
HTTP/1.1 200 OK
content-type: application/json; charset=utf-8
set-cookie: through-header=cookie set through reply.header; Secure; HttpOnly
set-cookie: through-plugin=cookie%20set%20through%20reply.setCookie
content-length: 86
Date: Wed, 10 Jan 2024 11:22:13 GMT
Connection: keep-alive
Keep-Alive: timeout=72

{"setCookieHeader":"through-header=cookie set through reply.header; Secure; HttpOnly"}
````

And server logs :
````
--> Set resp cookie through reply.setCookie
Read response cookies through reply.getHeader('set-cookie') undefined

--> Set resp cookie through reply.header
Read response cookies through reply.getHeader('set-cookie') through-header=cookie set through reply.header; Secure; HttpOnly

--------- server preSerialization -------------
Read response cookies through reply.getHeader('set-cookie') through-header=cookie set through reply.header; Secure; HttpOnly

--------- route preSerialization -------------
Read response cookies through reply.getHeader('set-cookie') through-header=cookie set through reply.header; Secure; HttpOnly

--------- server onSend -------------
Read response cookies through reply.getHeader('set-cookie') [
'through-header=cookie set through reply.header; Secure; HttpOnly',
'through-plugin=cookie%20set%20through%20reply.setCookie'
]

--------- route onSend -------------
Read response cookies through reply.getHeader('set-cookie') [
'through-header=cookie set through reply.header; Secure; HttpOnly',
'through-plugin=cookie%20set%20through%20reply.setCookie'
]

--------- server onResponse -------------
Read response cookies through reply.getHeader('set-cookie') [
'through-header=cookie set through reply.header; Secure; HttpOnly',
'through-plugin=cookie%20set%20through%20reply.setCookie'
]

--------- route onResponse -------------
Read response cookies through reply.getHeader('set-cookie') [
'through-header=cookie set through reply.header; Secure; HttpOnly',
'through-plugin=cookie%20set%20through%20reply.setCookie'
]
````

### Expected Behavior

I expect I can write/read response cookies through the plugin the "same way" I do through `reply.header('set-cookie', ...)`.
I would like the plugin to modify the `set-cookie` response header as soon as i call the `reply.setCookie` method.
I find it misleading to "hide" the cookies that will be sent until the onSend lifecycle step.