dev: impl serde traits on StarknetOsOutput

[EvolveArt]

As part of the integration in https://github.com/karnotxyz/madara-orchestrator we'd need the output to be (de)serializable for easy storage/reading.

Happy to provide a PR if necessary!

[odesenfans]

I implemented this feature in #245. This produces the following JSON format:

{
  "initial_root": "0x5594a2d89ad4eff183ea6a7f4d4bf247fb799f3db54bc6d94ea441e0c99a4ac",
  "final_root": "0x12b31d0ff0c0f5aa076d2e7039d2e19329a8a4a4ada68f42f2e0b6b8af304fd",
  "block_number": "0x7d1",
  "block_hash": "0x0",
  "starknet_os_config_hash": "0x5d4d0b87442f4c6c120e8d207e27c0e01796ad1e57c5323292ecaf655b53b05",
  "use_kzg_da": "0x0",
  "messages_to_l1": [],
  "messages_to_l2": [],
  "contracts": [
    {
      "addr": "0x1",
      "nonce": "0x0",
      "class_hash": null,
      "storage_changes": {
        "0x7c7": "0x42"
      }
    },
    {
      "addr": "0x1001",
      "nonce": "0x0",
      "class_hash": null,
      "storage_changes": {
        "0x27e66af6f5df3e043d32367d68ece7e13645cca1ca9f80dfdaff9013fddf0c5": "0xddec034b926f800",
        "0x723973208639b7839ce298f7ffea61e3f9533872defd7abdb91023db4658812": "0x1f67eee3d0800"
      }
    }
  ],
  "classes": {
  }
}

One of the limitations I can think of is that currently we do not parse messages in a meaningful way, we keep them as vectors of felts. I might get to do that sooner than later, TBD.

[odesenfans]

@EvolveArt let me know if this works for you, without an answer I'll merge it by end of day. We can make subsequent modifications, of course.