Rewritten or Restructure

### 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

We should reduce the methods to something that is necessary.
For example, `parts`, `files`, `file` does nearly the same but confusing.

We should either provide parsing method directly or `async iterator` for advanced user.

The current `request.body` object is not user friendly, which leads to multiple config (e.g. `keyValues`)

### Motivation

With the growth of feature and never removed deprecated functions.
The current codebase is messy and not user-friendly to beginner.

I found must people is not family with how stream works and ask for simple solution multiple times.
That's why I created [`fastify-formidable`](https://github.com/climba03003/fastify-formidable) and [`fastify-busboy`](https://github.com/climba03003/fastify-busboy).

I believe we can do something to provide a more generic interface and not fixed to a single parsing engine.
So, I created [`@kakang/fastify-multipart`](https://github.com/kaka-repo/fastify-multipart) and seek for advice.

### Example

_No response_