Allow putting `@key` on a field for the common single-key-of-single-field case

[pcmanus]

Currently, defining a key for an entity type is done with:

type User @key(fields: "id") {
  id: ID!
  x: Int
  y: Int
}

and this syntax certainly afford plenty of flexibility, in particular:

• it allows multiple separate keys.
• it allows compound keys, composed of more than on field, including nested ones.

However, the most common case is by far that of a single key on a single field. Some experimental sampling of real world data I did suggests that it is over 90% of cases.

As such, the proposal of this ticket is to introduce a convenience to simplify this most common case and to allow writing the example above more simply as:

type User {
  id: ID! @key
  x: Int
  y: Int
}

That is, to allow a @key without arguments directly on the one field that should be the key.

Technically, this would mean:

• changing the @key definition so that 1) the fields argument becomes optional and 2) it is allowed on FIELD_DEFINITION locations.
• add additional validation around @key usages so that:
  • the argument remains mandatory anywhere except when used on fields.
  • the argument is not accepted when used on fields.
  • @key on field is only accepted for fields of "leaf" types (and fields without arguments).
  • we cannot mix and match @key on fields and type for the same type.
  • we can use @key on field only on a single field of a type (or not; we could allow it on multiple fields as a way to declare multiple single-field keys, but happy to disallow it initially to avoid confusion).
  • whatever else I'm forgetting right now.

I reckon that this suggestion does not introduce anything you couldn't do before, and as such gives 2 ways to do the same thing, and it does require a bit of new validation to ensure we don't introduce confusing cases, but overall, it feels like an intuitive shortcut and save typing for the most common use of @key, so it feels way possibly worth.

Open to feedback, but as I would personally definitively use and enjoy such convenience, figured it was worth a ticket.