Support for bigint

### 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

5.6.1

### Plugin version

8.2.0

### Node.js version

22.18.0

### Operating system

Linux

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

ArchLinux

### Description

If you pass the object with bigint values, it gives you an error.

### Link to code that reproduces the bug

/

### Expected Behavior

Successfully serialize bigint.
You can use replacer for JSON.stringify at least.