Support manual hook injection

### 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

I have multiple hooks during the preValidation step including hooks that modify the content or run a signature check on the content. Because the hook is registered at the instance boot and not at the route level i cannot decide on the hooks order.

https://github.com/fastify/fastify-multipart/blob/83aea9789d45a6222041a4fa33fd9535a179f4b9/index.js#L70

Would it be possible to put the hook in a dedicated exported function and add a new parameter to allow to manually load the hook via the function?

### Motivation

_No response_

### Example

_No response_