Composite resolver keys

[gmac]

Adds support for composite resolver keys, for example:

interface FieldOwner {
  id: ID!
  type: String!
}
type CustomField {
  owner: FieldOwner!
  key: String!
  value: String
}
input CustomFieldLookup {
  ownerId: ID!
  ownerType: String!
  key: String!
}
type Query {
  customFields(lookups: [CustomFieldLookup!]!): [CustomField]! @stitch(
    key: "owner { id type } key",
    arguments: "lookups: { ownerId: $.owner.id, ownerType: $.owner.type, key: $.key }"
  )
}