Default value by constant and expression in CRUD, models, faker etc

Example: If x-db-default-expression is present then ensure it is present as `\yii\db\Expression(...)` in model validation `rules()`.

```
x-db-default-expression
x-db-default
```

Pulled from https://github.com/cebe/yii2-openapi/issues/122

Also part of https://github.com/php-openapi/yii2-openapi/issues/10