Support for Apollo Upload scalar

[huboneo]

Apollo server v2 supports file uploads as a multipart/form-data request using their Upload scalar https://www.apollographql.com/docs/apollo-server/data/file-uploads/

Right now Sofa seems to hardcode all POST content-types to application/json, could we change this to check for Upload scalar and if so use multipart/form-data content?

[Urigo]

@huboneo sounds interesting! Care to try to contribute it? You can start with something simple and we could help you with it!