AIP-122: Disallow embedding other resources as fields in a resource

[noahdietz]

AIP-122 outlines how a resource should refer to another resource in its own message definition, using a string field representing the other resource's name, with a google.api.resource_reference annotation to document the format.

It does not, however, call out the anti-pattern of embedding a resource reference as message type field. For example:

message Publisher {
  option (google.api.resource) = {
    // ...
  };

  // ...

  // Authors associated with the Publisher, where Author is also a resource.
  repeated Authors authors = 2;
}

Instead, the field should be:

  repeated string authors = 2 [
    (google.api.resource_reference).type = "library.googleapis.com/Author"
  ];

I propose that AIP-122 should call out this anti-pattern and correction explicitly.

[noahdietz]

cc: @ibelle