this addition allows to provide a scoped collection of records when defining a resource relation.

Example:

```
class Event < ApplicationRecord
  belongs_to :location

  def self.alchemy_resource_relations
    {
      location: {
        attr_method: "name",
        attr_type: "string",
        collection: Location.where(...)
      },
    }
  end
end
```

In the view the user will see the scoped collection of locations in the select box.

In case `collection` is not passed

  * `Location.all` will be used
  * `class_name` on the association is used if it's defined