Add decoding commands to dapf

[mendess]

This allows for easier decoding of DAP outputs

Using this

cargo run --bin dapf -- decode ./path/to/collection_output collection \
    --vdaf-config '{"prio3": { "sum": { "bits": 8 } } }' \
    --task-id "8TuT5Z5fAuutsX9DZWSqkUw6pzDl96d3tdsDJgWH2VY" \
    --hpke-config-path ./hpke-config.json \
    --report-count 2

and assuming that ./hpke-config.json has the hpke config used by the collector, this command can decode and show the aggregated value

example output:

{
  "decrypted": {
    "u128": 42
  },
  "original": {
    "encrypted_agg_shares": [
      {
        "config_id": 183,
        "enc": "e1fda76d2c9d905606886147e19ca0c7cdf6aea4d933b265db986fe7f679f86c",
        "payload": "6fdef070cbecd13e010e8bc87c8e77ab36908e37344595f78215b398126fa062"
      },
      {
        "config_id": 183,
        "enc": "413f15cc4bb3bc190175adb792bb41da9e4052adca39a59db09cb00ca62c4552",
        "payload": "ac653775e93bf82d4e7b5807fed8b81508ebaacd180df47e0f4ecf4b23479ddb"
      }
    ],
    "interval": {
      "duration": 3600,
      "start": 1727715600
    },
    "part_batch_sel": {
      "fixed_size_by_batch_id": {
        "batch_id": "544ac0e120712ac6ac3dd50ae3d3bee70c0ffb1e10a396cb250c66d1afc8cbf6"
      }
    },
    "report_count": 1
  }
}

[thibmeu]

various could use some precision.
possibly also add an example command and how to retrieve/generate the payload from a leader/helper, maybe with dapf itself