Implement a method to_document on QueryPlan to create an ExecutableDocument.

Here is an example of how the ExecutableDocument should look like when rendered:

{
  QueryPlan {
    Sequence {
      Fetch(service: "SubgraphA") {
        updateInAOne: updateFooInA {
          id
          bar
        }
      }
      Fetch(service: "SubgraphB") {
        updateInBOne: updateFooInB {
          id
          baz
        }
      }
    }
    Fetch(service: "SubgraphA") {
      updateInATwo: updateFooInA {
        id
        bar
      }
    }
  }
}

Technical Requirements

• Implement it as a method to_doc on QueryPlan<A> to create an ExecutableDocument.
• Add unit tests for some sample QueryPlans in model.rs