JSON Schema Definitions not work

### Prerequisites

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

` "@fastify/swagger": "7.4.1",`

### Issue

Hi Guys, I add new schema with addSchema in my fastify like this :

```
{
$id: 'http://foo/common.json',
type: 'object',
definitions: {
foo: {
$id: '#address',
type: 'object',
properties: {
city: { type: 'string' }
}
}
}
}
```

and used in my route schema

```
schema: {
body: {
$ref: 'http://foo/common.json#/definitions/foo'
}
},
```

its work in fastify no error found, but error in swagger

Could not resolve reference: Could not resolve pointer: /definitions/def-0/definitions/foo ddoes not exist in document

and when i check /documentation/json there is no my definitions inside -> definitions -> def-0

and models only {}

Start with the addSchema usage and the route schema shown in the issue, then inspect the generated /documentation/json output and the unresolved reference. Compare the reported definitions structure with the linked pull request #676; done means the schema reference resolves and the generated documentation includes the expected model definitions.

Written by the indexing model from the issue text.