Support GraphQL uploads via multipart form spec

[gmac]

This adds multipart form extraction to HttpExecutable, which enables GraphQL uploads. Resolves #101, as proposed by @mikeharty.

Extractor

The following variables...

a = Tempfile.new
b = Tempfile.new

{
  "input"=>{"file"=>a, "files"=>[a, b]},
  "inputs"=>[
    {"file"=>a, "files"=>[a, b]}, 
    {"file"=>a, "files"=>[a, b]}
  ]
}

Extract into the following multipart form mapping...

{
  "map"=> {
    "0"=> [
      "variables.input.file",
      "variables.input.files.0",
      "variables.inputs.0.file",
      "variables.inputs.0.files.0",
      "variables.inputs.1.file",
      "variables.inputs.1.files.0"
    ],
    "1"=> [
      "variables.input.files.1", 
      "variables.inputs.0.files.1", 
      "variables.inputs.1.files.1"
    ]
  },
  "0"=>"A",
  "1"=>"B",
}

[mikeharty]

Hey @gmac, I wanted to say thanks for getting this in! I didn’t intend for this to be a drive-by feature request, I had a shift in priorities after the holidays and was totally preoccupied. Thanks for the help with this and the other features we discussed, the project I was working on is now a running a Supergraph in production built on this. 👍🏻

[gmac]

@mikeharty awesome! This was a cool feature and I learned a lot about GraphQL upload building it. Thanks for the prototype and introduction. Great to hear that it’s running in the wild.