WIP: 28 handle file upload with graphql

[Asone]

This could be a base to provide a solution to the enhancement request in juniper

A few improvements should be done before merging the branch :

• [x] Files Vec should be populated with TempFile instances instead of simple Strings that provide path
• [x] Configure mod as a library
• [x] Implement demo combining Paywall and file uploads

On further steps the lib could be improved in order to be shared to Juniper Community for merge or publication with :

• Environment configuration handling
• Unit tests
• Documentation