Get session ID from header instead of cookie

### 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

As the title say, a way to allow for other ways to retrieve the session id from the client (and also send it back), maybe not only with headers, but creating a interface that the user could implement any custom logic to it.

### Motivation

I would like to use sessions to store data in my API, accessing it from places that are not a browser, where handling cookies are not as convenient as just a plain header

### Example

```js
app.register(fastifySession, {
secret: 'a secret with minimum length of 32 characters',
// Maybe you guys have a better name suggestion for this parameter
sessionId: {
retrieve(request, cookieOptions) {
return request.headers['session-id'];
},
set(reply, sessionId, cookieOptions) {
reply.headers('session-id', sessionId);
}
}
});
```