New rule: enforce a single mutation field

[dotansimha]

By default, should work for the root level of Mutation:

mutation {
   something # valid
}

mutation {
   something # valid
   somethingElse # invalid
}

But also should be configurable to allow choosing the nesting level:

mutation {  # valid
   mutationGroup {
      doSomething
   }
}

mutation { #invalid
   mutationGroup {
      doSomething 
      doSomethingElse
   }
}