Allow join types in many-to-many to not have a primary key

[ramnivas]

Consider Concert -> [ConcertArtist] <- Artist. Currently, we require that all Postgres types carry a field marked as @pk, but in case of many-to-many relations, it doesn't make much sense for the joining entity to carry a primary key.

type Concert {
   @pk id: Int ...
   title: String
   concertArtists: Set<ConcertArtist>?
}

type Artist {
   @pk id: Int ...
   name: String
   concertArtists: Set<ConcertArtist>?
}

type ConcertArtist {
   @unique("participation") concert: Concert
   @unique("participation") artist: Artist
   role: String
}

We should examine if there is a broader use case for pk-less types ("value types").