More updates to dynamic schema implementation

[rmosolgo]

• Support multiple interface implementations
• Sort interfaces in introspection response
• Fix a call to arguments
  • why wasn't this caught by the linter?
  • Add a test for this case
• Mutations: build up the input type and payload type as the mutation is defined, instead of defining them one-off when the mutation is hooked up to the field. (This way, updates to the mutation class will update the derived types, too.)

Follow up to #3651 , part of #3274