Please add support for Rails polymorphic assotiations.

The desired outcome would be:

```
FixtureBot.define do |t|
  t.role(:coach) do
    resource coaches(:boti_coach)
  end
```

instead of:

```
FixtureBot.define do |t|
  t.role(:coach) do
    # resource_id Coach.first.id
    # resource_type Coach.first.type
  end
```
