Support for Apollo Upload scalar

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?

The issue does not name a file or test; start by tracing where POST content types are selected and compare the behavior with Apollo Server's Upload scalar documentation. Done means requests using Upload are sent as multipart/form-data instead of application/json, with existing POST behavior preserved.

Written by the indexing model from the issue text.