how to disable "belongs-to" validator?

Hi, thank you for your great validation tool - ember-cp-validation. I have a question: how to disable "belongs-to" validator?
I have a model "user", which has "belongs-to" relationship to model "account". In one case I need to validate only "user" and to not validate "account" model, in other case need to validate both.

```
// user model
import Validations from "../cp-validations/models/user"
import DS from "ember-data";

export default DS.Model.extend(Validations, {
lastName: DS.attr("string"),
account: DS.belongsTo("account")
});
```

```
// account model
import Validations from "../cp-validations/models/account"
import DS from "ember-data";

export default DS.Model.extend(Validations, {
login: DS.attr("string")
});
```
The files with validation rules:
```
// "../cp-validations/models/user"
import { validator, buildValidations } from "ember-cp-validations";

export default buildValidations({
lastName: [
validator("presence", { presence: true, message: "* required" }),
validator("length", { max: 50, message: "Max {max} characters" })
],
account: validator("belongs-to")
});
```

```
// "../cp-validations/models/account"
import { validator, buildValidations } from "ember-cp-validations";

export default buildValidations({
login: [
validator("presence", { presence: true, message: "* required" }),
validator("length", { max: 60, message: "Max {max} characters" })
]
});
```

I've tried to use `account: validator("belongs-to", { disabled: true })`, but in this case calling `user.validate().then(....)` the "then" never gets fired.
The 'disabled' attribute works fine for regular validators like "length", "presence", etc. But does not work for "has-many" and "belongs-to" validators

### Environment

- Ember Version: 3.5.0
- Ember CLI Version: 3.5.0
- Ember CP Validations Version: 3.5.3

Reproduce the case using the validation rules in ../cp-validations/models/user and ../cp-validations/models/account, starting with user.validate(). Compare the behavior of disabled regular validators with the belongs-to and has-many validators. Done means validation resolves when relationship validation is disabled and still validates account when it is enabled.

由索引模型根据 Issue 内容生成。