Different cookie secret based on request

### Prerequisites

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

### 🚀 Feature Proposal

We have a multi-tenant site where the tenant name is in the url like `tenant1.site.com`, `tenant2.site.com` etc.

I would like each tenant to have a different session secret in fastify session which seems to boil down to the cookie secret. Today it is possible to implement a custom signer, however it does not receive the request as a parameter so it is not possible to use different secrets for sign/unsign per tenant.

I think if the custom signer could receive request as a parameter this could be solved.

```
sign: (value, req) => { ... }
```

However, maybe there is a better already solution to my particular scenario?

### Motivation

Support for multi tenancy.

### Example

```ts
fastify.register(require('@fastify/cookie'), {
secret: {
sign: (value, req) => {
// get tenant from req and fetch corresponding secret
// sign cookie with the tenant's secret
return signedValue
},
unsign: (value, req) => {
// get tenant from req and fetch corresponding secret
// unsign cookie with the tenant's secret
return {
valid: true, // the cookie has been unsigned successfully
renew: false, // the cookie has been unsigned with an old secret
value: 'unsignedValue'
}
}
}
})
```