Support Firebase / GCP cloud functions

### 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

Support serverless firebase and GCP

### Motivation

Currently we can't use this plugin with [serverless](https://fastify.dev/docs/latest/Guides/Serverless/) GCP (or Firebase).

As explained here https://github.com/fastify/fastify/issues/946#issuecomment-766319521 and [in the doc for firebase](https://fastify.dev/docs/latest/Guides/Serverless/#add-custom-contenttypeparser-to-fastify-instance-and-define-endpoints), we need to use `req.rawBody` in order to parse files from multipart.

### Example

_No response_