[Feature] fastify-multipart interceptors

[maronfranc]

Feature Request

Platform Fastify FileInterceptor, FilesInterceptor, AnyFilesInterceptor and FileFieldsInterceptor

Is your feature request related to a problem? Please describe.

There is a fastify-multer pull request and @emagnier asked to consider adding a fastify-multipart version.

Describe the solution you'd like

I implemented a fastify-multipart wrapper that works in a similar way as platform-express interceptors.
I'd like to create a PR, so we can discuss if it is a good solution.

Teachability, Documentation, Adoption, Migration Strategy

The documentation will be almost same as it is, the main difference is that the uploaded file interface is a modified version of fastify-multipart MultipartFile interface.

What is the motivation / use case for changing the behavior?

File upload for platform-fastify.

[kamilmysliwiec]

Feel free to create a PR for this issue if you have a working version ready 🙌 Just a heads-up however, we cannot promise whether we'll merge it or not just yet (the decision if we want to provide fastify-multipart integration right from the platform-fastify package hasn't been made).