Better support for boundary types without resolvers

[gmac]

As pointed out by @thomasmarshall, it's actually okay for a boundary type without a resolver query to contain non-key fields, as long as those fields are resolvable elsewhere. For example:

# A
type T { id:ID! name:String } 
type Query { a(id: ID!):T @stitch(key: "id") }

# B
type T { id:ID! size:Float } 
type Query { b(id: ID!):T @stitch(key: "id") }

# C
type T { id:ID! size:Float name:String } 
type Query { c:T }

In the above, schema C does not provide a resolver query for T so it acts as an outbound-only type. This is okay because all of its fields can be resolved elsewhere when needed.