Add individual buying/selling statistics in stats endpoint

[Kaptard]

This will likely require breaking changes. A sensible schema may look like this:

{
  item: "itemname", // key used to be called title, but for consistency item is preferred
  buying: {
    offers: {
       count: Number,
       percentage: Number,
    },
    components: [/*...*/] // original components array, but offers {} like above instead of supply/demand
  },
  selling: {/* same as buying */},
  combined: {/* same as original, but no supply/demand keys, use offers instead, offers.percentage is 100%*/}
}

Might have missed something, so it's best to look at other possibilities too.

[Nakroma]

For now in v1/items/statistics_new