Add e-commerce transaction tracking

[aeirola]

Since the data is rather complex, we might need to check how the SDKs and piwik actually behaves in these situations. Preferably the result should be identical on both platforms

trackEcommerceOrder(
  orderId: string,
  grandTotal: number,
  subTotal?: number,
  tax?: number,
  shipping?: number,
  discount?: number,
  items?: Array<{
    sku: string,
    name?: string,
    category?: string,
    price?: number,
    quantity?: number,
  }>,
)

See:

• https://developers.piwik.pro/en/latest/tracker/tracker_object.html#trackEcommerceOrder
• https://developers.piwik.pro/en/latest/sdk/Piwik_PRO_SDK_for_Android.html#tracking-ecommerce-transactions
• https://developers.piwik.pro/en/latest/sdk/Piwik_PRO_SDK_for_iOS.html#tracking-ecommerce-transactions