use static key names in index.json for easy unmarshalling

The current index.json format is rather annoying to unmarshal because the key names are not known until runtime (e.g. "v21.06", and so on).

I came up with the following, that seems to be much easier to unmarshal, at least using Go's json lib:

example

{
  "versions": [
    {
      "name": "edge",
      "devices": [
        {
          "name": "motorola-harpia",
          "interfaces": [
            {
              "name": "sxmo",
              "images": [
                {
                  "name": "20210929-0323-postmarketOS-edge-sxmo-1.5.1-motorola-harpia-mainline-modem.img.xz",
                  "time": "2021-09-29T03:23:00.000Z",
                  "size": 358629100,
                  "url": "https://images.postmarketos.org/bpo/edge/motorola-harpia/sxmo/20210929-0323/20210929-0323-postmarketOS-edge-sxmo-1.5.1-motorola-harpia-mainline-modem.img.xz",
                  "sha256": "f88d5ea98683cd4f8beab8050fbe8f35909f03f73b4efeed9b398aa858a5b048",
                  "sha512": "a2af2216fe6c42e4f7adbd06827c568269d375a84fd02039b858510e9816fa467d64bd1cdcc84cb1e75d153e53a5f419c6caaa332b12f2265ef7c03814e10b6b"
                },
                {
                  "name": "20210915-0302-postmarketOS-edge-sxmo-1.5.1-motorola-harpia-mainline-modem.img.xz",
                  "time": "2021-09-15T03:02:00.000Z",
                  "size": 384512544,
                  "url": "https://images.postmarketos.org/bpo/edge/motorola-harpia/sxmo/20210915-0302/20210915-0302-postmarketOS-edge-sxmo-1.5.1-motorola-harpia-mainline-modem.img.xz",
                  "sha256": "754813870a9c1a702df4000000670c97284fc867eabf0230734f6b66c85a55d7",
                  "sha512": "a522a0f4926e409899c63321a061e163290da79a985a998834e0988aa60491e9368cdf8de7bde729479c6727580eb0f243ce6f02b259bf0647e19bdc720eb376"
                }
              ]
            },
            {
              "name": "phosh",
              "images": [
                {
                  "name": "20210929-0248-postmarketOS-edge-phosh-12-motorola-harpia-mainline-modem.img.xz",
                  "time": "2021-09-15T03:02:00.000Z",
                  "size": 428594436,
                  "url": "https://images.postmarketos.org/bpo/edge/motorola-harpia/phosh/20210929-0248/20210929-0248-postmarketOS-edge-phosh-12-motorola-harpia-mainline-modem.img.xz",
                  "sha256": "3f56e5cedb2b58c56c76865006f22bdcefe3aaf941b32694aad1b7cadc47a40c",
                  "sha512": "11011d1c8796a87e77a813d23ba6ca4d04514d9549c9222247fa135a4e3820efa214408d98383f06f07603791538da2c7e83ca22fffd014d0e4b3ad22da2426f"
                }
              ]
            }
          ]
        },
        {
          "name": "pine64-pinephone",
          "interfaces": [
            {
              "name": "plasma-mobile",
              "images": [
                {
                  "name": "20210929-0707-postmarketOS-edge-plasma-mobile-3.3-pine64-pinephone.img.xz",
                  "time": "2021-09-29T07:07:00.000Z",
                  "size": 686088380,
                  "url": "https://images.postmarketos.org/bpo/edge/pine64-pinephone/plasma-mobile/20210929-0707/20210929-0707-postmarketOS-edge-plasma-mobile-3.3-pine64-pinephone.img.xz",
                  "sha256": "ef93edd31d84dea417c5d9d49d5ea5cbc92bd0429240543e1962db931d60c37e",
                  "sha512": "458a85f8dd4c2b72840466a1d991b30daad1a69ec7c36d5615a03504ae5862b90cc3db09d7313717486f72c6def341b7d30a666618876bd5f4b1e3dbbe560405"
                },
                {
                  "name": "20210929-0707-postmarketOS-edge-plasma-mobile-3.3-pine64-pinephone-installer.img.xz",
                  "time": "2021-09-29T07:07:00.000Z",
                  "size": 897163876,
                  "url": "https://images.postmarketos.org/bpo/edge/pine64-pinephone/plasma-mobile/20210929-0707/20210929-0707-postmarketOS-edge-plasma-mobile-3.3-pine64-pinephone-installer.img.xz",
                  "sha256": "786b522875b278827913e7466532352b85fb50056a170ea93d7823b5a42b6fd9",
                  "sha512": "b43a85fec089c91384d528ea76a9e9e13f253fc13f42c07c4899709658d63eff5388efa42ad6b84836ffa69ae0bf12298e1f177654ab97fde2e5ee7206d4bc21"
                }
              ]
            }
          ]
        }
      ]
    }
  ]
}

@ollieparanoid you did the original implementation of this, so wanted your thoughts on this change before I do the work to, well, change it :D