Implementing Composite Graph Api

[dscataglini]

I'd like to implement the composite graph api https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/resources_composite_graph_introduction.htm

Something like this

client.composite_graph do |graphs|
  graphs.add("graph1") do |subrequest|
    subrequest.create("Account", "acc_1", {name: "main account"})
    subrequest.create("Contact", "contact", {firstName: 'Joe', Email: 'test@salesforce.com', AccountId: '@{acc_1}'})
    …
  end 
  graph.add("graph2") do |subrequest|
    …
  end
end