[product-categories]: add option to send all of them in separate lists

[rrickgauer]

if the user adds an optional flag to the url query: ?seperate_list=true

then return the json body as 3 lists:

1. major
2. minor
3. sub

[rrickgauer]

The response body should look like:

{
    major: [
        {
            id: id,
            name: name
        },
    ],

    minor: [
        {
            id: id,
            parent_id: major_category_id,
            name: name,
        },
    ],

    sub: [
        {
            id: id,
            parent_id: minor_category_id,
            name: name,
        },
    ],
}