ReferencesMany relation type

A ReferencesMany relation embeds an array of foreign keys to reference other objects. 

An example model instance:

```js
{
  id: 1,
  name: 'John Smith',
  accounts: [
    "saving-01", "checking-01",
  ]
}
```

See also LB 3.x docs: https://loopback.io/doc/en/lb3/Embedded-models-and-relations.html