Disable accumulating files in memory while using attachFieldsToBody

### 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

Allow a flag in options to disable accumulation of files in memory, and let the caller handle the files themselves

Currently, if you want to use attachFieldsToBody, you're forced to either allow the library to accumulate files in memory, or provide an onFile function... Which also forces you to process the file (whether by streaming or accumulating).

I'd like to use the attachFieldsToBody feature, but I don't want any of my files accumulated or processed before the api function, because I need to handle them separately later and do streaming.

### Motivation

attachFieldsToBody is a useful feature, and I'd rather not have to re-implement it so I can disable the accumulation

### Example

`fastify.register(require('@fastify/multipart'), { attachFieldsToBody: 'keyValues', disableAccumulation: true })`