feature request: support multipart requests

[ddelgrosso1]

Currently it does not appear that Gaxios supports the ability to send multipart requests. As maintainers of google-cloud/storage we would like to be able to move away from teeny-request (which does support multipart requests) as well as add the ability to do batch deletes which require multipart requests. It would be nice to add multipart requests to this library.

[mpiorowski]

The same request, the documentation says that when you send data as ReadableStream, it should set the corrent content-type. I send ReadStream, so i would expect it to work, but it doesn't. To be more precise i would expect this to work:

data: fs.createReadStream(...)

[mpiorowski]

Or even when i try to put FormData into data, still not working.

[ddelgrosso1]

I have started implementing this so reassigning myself.