Proposal to add more details to shopping cart contents and shipping option descriptions

1. We would like to add ability to specify shopping cart content images in Payment Request.
2. We would like to add ability to specify both the primary and the second label for shipping options. This would allow distinguishing between the shipping speed ("2-day") and the expected delivery date ("Sunday, May 5").

For example:

new PaymentRequest(paymentMethods, {
  total: {label: "Total", amount: {currency: "USD", value: "50"}},
  displayItems: [{
    label: "Hat",
    icons: [{
      src: "hat64.webp",
      sizes: "64x64",
      type: "image/webp"
    },{
      src: "hat64.png",
      sizes: "64x64"
    }, {
      src: "hat128.png",
      sizes: "128x128"
    }],
    amount: {currency: "USD", value: "50"}
  }],
  shippingOptions: [{
    id: 'twoday',
    label: '2-day',
    sublabel: 'Sunday, May 2',
    amount: {currency: "USD", value: "0"}
  }]
}, {requestShipping: true});

cc @fengrui129 @zkoch @DurgaTheHutt

Start with the Payment Request API specification, focusing on displayItems and shippingOptions in the issue's example. Determine how item images and primary and secondary shipping labels should be represented, then document the proposal's intended behavior and compatibility considerations.