Pino object logger option gets cloned after #568

### 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

4.10.2

### Plugin version

5.7.0

### Node.js version

18.12.1

### Operating system

macOS

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

13.0.1

### Description

After #568 was merged, the new behavior of `deepmerge` clone the `options` object and tries to clone any instance of Pino we pass as option, modifying it into a simple object so that all the Pino settings are lost.

### Steps to Reproduce

Given the following fastify startup module:

```javascript
import pino from 'pino';

export default async function (fastify, options) {
fastify.get('/', async (req, reply) => ({ hello: 'world' }));
}

const logger = pino({
transport: {
target: 'pino-pretty',
},
});

export const options = {
logger,
};
```

When using fastify-cli like this:

node_modules/.bin/fastify start --options a.js

I get this output:

{"level":30,"time":1670931640310,"pid":68940,"hostname":"Stefanos-MacBook-Air.local","msg":"Server listening at http://[::1]:3000"}
{"level":30,"time":1670931640311,"pid":68940,"hostname":"Stefanos-MacBook-Air.local","msg":"Server listening at http://127.0.0.1:3000"}

### Expected Behavior

The expected output would be something like this instead:

[12:41:03.421] INFO (69034): Server listening at http://[::1]:3000
[12:41:03.422] INFO (69034): Server listening at http://127.0.0.1:3000

I would tend to think that `deepmerge` should be modified in order to clone by reference any type of object that is not a plain JS object, but I'd appreciate any feedback in order to try and provide a patch for this.