Support sort parameters that do not relate to a column

[lindyhopchris]

At the moment, the package only supports sort parameters that directly relate to a field in the schema (i.e. a database column) - via the sortable() method.

It should also be possible to add sort parameters that don't relate to a field. Need to add this in before 1.0, as it is supported in the cloudcreativity/laravel-json-api package.

As per the sortByLikes example here:
https://laravel-json-api.readthedocs.io/en/latest/fetching/sorting/#implementing-sorting

And also it should be possible to do things like order-by sub-queries:
https://laravel-news.com/eloquent-subquery-enhancements

[lindyhopchris]

Also:

• need to make sure that default sorting is implemented and working, because it exists in the legacy package;
• need to make sure the docs has a schemas/sorting chapter, like the other query parameter chapters.