Nested entry GraphQL support

[brandonkelly]

Adds support for querying and mutating nested entries directly, rather than just through the top-level entry.

query {
  contactMethodsFieldEntries(ownerId: 253) {
    ...on contactMethod_Entry {
      id
      label
      sortOrder
      fieldHandle
      fieldId
      ownerId
    }
  }
}

mutation {
  save_contactMethodsField_contactMethod_Entry(
    ownerId: 253
    label: "Email"
    methodValue: "support@craftcms.com"
  ) {
    id
    label
    methodValue
  }
}