Validate array of objects

### Environment
- Ember CP Validations Version: 3.5.1

How would I go about validating an array of objects from the model? Validating profile.address nested attrs works fine. Not sure how to go about validating name and email for each friend?

I have the following example:

Data:
```
profile: {
friends: [{
name: 'bob',
email: 'bob@email.com'
}, {
name: 'sue',
email: 'sue@email.com'
}],
address: {
street: '10 Bash St',
town: 'Dundee'
}
}
```

```
import Model from 'ember-data/model';
import attr from 'ember-data/attr';

import { computed } from '@ember/object';

import { validator, buildValidations } from 'ember-cp-validations';

const Validations = buildValidations({
name: [
validator('presence', true),
validator('length', {
min: 4
})
],
'profile.address.street': [
validator('presence', true),
validator('length', {
min: 4
})
],
'profile.address.town': [
validator('presence', true),
validator('length', {
min: 4
})
]
});

export default Model.extend(Validations, {

// Attributes

name: attr('string'),
// custom transform
profile: attr('object', {
defaultValue() {
return {
friends: [],
address: {}
}
}
}),

// Associations

// Computed Properties

isFormValid: computed.alias('validations.isValid')

});
```

Start with the model example in the issue, especially the existing nested profile.address validations and the profile.friends array. Check the validation API and related tests for nested object or array-item validation, then establish whether the expected outcome is documented configuration or a library change that validates each friend's name and email.

由索引模型根据 Issue 内容生成。