Add additional attributes to support item-backed add-ons

[joannasese]

To create item-backed add-on:

$add_on = new Recurly_Addon();
$add_on->plan_code = $plan->plan_code;
$add_on->item_code = $item->item_code;
$add_on->unit_amount_in_cents->addCurrency('USD', 400);
$add_on->create();

To get item-backed add-on:

Recurly_Addon::get($plan->plan_code, $item->item_code);
# or
Recurly_Addon::get($plan->plan_code, $add_on->add_on_code);

To update item-backed add-on:

$add_on->unit_amount_in_cents['USD'] = 200;
$add_on->update();

When add-on has an associated item, only the following attributes are updatable:

unit_amount_in_cents
default_quantity
display_quantity_on_hosted_page
optional

[bhelx]

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