Add a basic file(s) interceptor/decorator for Fastify

[xegulon]

Is there an existing issue that is already proposing this?

• I have searched the existing issues

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

I have an issue while trying to use nestjs with Express and Fastify interchangeably, because nestjs/platform-fastify does not implement a decorator/interceptor for multipart form data files.

Describe the solution you'd like

The counterpart of FileInterceptor, UploadedFiles for nestjs+fastify

Teachability, documentation, adoption, migration strategy

Adapting the docs for https://docs.nestjs.com/techniques/file-upload

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

It's an addition, not a modification of something already existing

[jmcdo29]

There's an open PR to integrate fastify-mutler that's been open for a while. There's also @nest-lab/fastify-multer to bring a FileInterceptor for fastify, and soon to be a @nest-lab/multer that integrates any multer so long as the user provides the multer instance. I need to create that last one though