schema: add output "is_required" field

[oliver-sanders]

At present there are two fields available on task outputs, "key" and "value".

It would be useful to have "is_required" too so that Cylc interfaces can highlight required outputs that have not been completed.

query {
  workflows {
    tasks {
      runtime {
        outputs {
          key
          value
        }
      }
    }
  }
}