Implement file upload for package parameter in put_package

[jannic]

I noticed that put_package is still not usable because the upload of the package contents isn't implemented.

Of course this merge request is of limited usefulness because the fastly-api source code is generated. But perhaps it's still helpful when you improve the code generator?
For that reason I also didn't bother to replace the unwrap call with proper error handling.

[Integralist]

👋🏻 Thanks @jannic

I appreciate you providing this working example. I'll take a look at the code-generator template and see if we can work this into there without too much disruption 👍🏻