Provide support for lazy-loading relations at access time

[ocratravis]

Is your feature request related to a problem? Please describe...

Suppose that I have a Company model, which has many Employee models.

Currently, I can't do this:

const company = Company.find(123)
console.log(company.employees) // outputs an empty array, even if company 123 does have employees

Instead, I have to make sure that the employees relation gets loaded as part of the initial query:

const company = Company.query().with(['employees']).whereId(123).first()
console.log(company.employees) // outputs the related employees, as expected

If the code which needs to display the employees is in a different part of the app than the code which initially loads the company, this behavior creates a tight coupling between the two parts of the app.

Describe the solution you'd like

When I try to access a relation that hasn't yet been loaded, I would like Vuex ORM to lazy-load the related model(s). This is consistent with other ORMs that I've used, such as Laravel Eloquent.

const company = Company.find(123)
console.log(company.employees) // Vuex ORM sees that this relation hasn't yet been loaded, and lazy-loads it from the store