Adds the ability to itemize proration discount amounts

// Line Item
{
  "proration": true,
  "amount": 700,  // Net: 7 USD after discounts (10 USD - 1 USD item - 2 USD group)
  "discount_amounts": [
    // Item-level discount missing entirely
    {"amount": 0, "discount": "di_group_2off"}  // Group-level shows ID but no amount
  ]
}

// Invoice Item
{
  "proration": true,
  "amount": 700  // Net amount only
}

// Invoice
{
  "total_discount_amounts": [
    {"amount": 0, "discount": "di_group_2off"}  // Missing item-level proration discount
  ],
  "subtotal": 700  // Adds proration amount after all discounts
}

// POST /v1/subscriptions
{
   // ... omitting irrelevant fields ...
   billing_mode: {
      type: "flexible",
      flexible: {  // New parameter: only available when billing mode type is flexible
         proration_discounts: "itemized"  // Default value is "included"
      }
   }
   // ... omitting irrelevant fields ...
}

// Line Item
{
  "proration": true,
  "amount": 1000,  // Gross: 10 USD before discounts
  "discount_amounts": [
    {"amount": 100, "discount": "di_item_10pct"},  // Item-level discount now visible
    {"amount": 200, "discount": "di_group_2off"}   // Group-level with actual amount
  ]
}

// Invoice Item
{
  "proration": true,
  "amount": 1000,         // Gross amount
  "net_amount": 700,      // New property: net amount after both discounts
  "proration_details": {  // New property: populated when the invoice item is a proration
    "discount_amounts": [
      {"amount": 100, "discount": "di_item_10pct"},
      {"amount": 200, "discount": "di_group_2off"}
    ]
  }
}

// Invoice
{
  "total_discount_amounts": [
    {"amount": 100, "discount": "di_item_10pct"},  // Now includes item-level proration discount
    {"amount": 200, "discount": "di_group_2off"}   // Now includes group-level proration discount
  ],
  "subtotal": 9000  // Includes proration amount after item-level discounts
}