User DataQueryable.contains() for getting many-to-many associations

**Description**
`DataQueryable.contains()` may be used for filtering items based on a many-to-many association e.g. get users that belong to Administrators group.

```js
const group = await context.model('Group').where('name').equal('Administrators').getItem();
const users = await context.model('User').where('groups').contains(group).getItems();
```


