Abstract directive definition

[nohponex]

As a directive we define

• JSON API's page, sort, fields and filter
• additional internal arguments
• any other future extension to JSON API

Our goal is remove argument order restrictions on resource model get and getById calls allowing

• future expansion
• omission of directives with null value (unset/unused)

Proposal is to have an interface class named IDirective each of the above MUST implemented.

get method signature MUST change to:

function get(IDirective ...$directives)

An execution example:

$users = Users::get(
    new Sort('name'),
    new Page()
);

Internally in get method value of given directive if any can be found like this:

$page = array_filter(
    $directives,
    function ($directive) {
        return get_class($directive) === Page::class;
    }
) ?? null; // or it can be get default instead of null 

NOTES:

• The signature proposal is not final, it should probably not restrict additional arguments if needed in future
• It's important to solve the issue when a relationship inclusion requires the existence of a resource attribute (forced fields)