Refactor: rename `peer.updated` field in torrent detail endpoint

[josecelano]

pub struct TorrentPeer {
    pub peer_id: PeerId,
    pub peer_addr: SocketAddr,
    #[serde(serialize_with = "ser_instant")]
    pub updated: std::time::Instant,
    #[serde(with = "NumberOfBytesDef")]
    pub uploaded: NumberOfBytes,
    #[serde(with = "NumberOfBytesDef")]
    pub downloaded: NumberOfBytes,
    #[serde(with = "NumberOfBytesDef")]
    pub left: NumberOfBytes,
    #[serde(with = "AnnounceEventDef")]
    pub event: AnnounceEvent,
}

@WarmBeer , would not it be nice to rename the updated field like this:

#[serde(serialize_with = "ser_instant")]
#[serde(rename(deserialize = "updated_milliseconds_ago", serialize = "updated_milliseconds_ago"))]
pub updated: std::time::Instant,

To get a json like this:

{
  "info_hash": "4beb7001cb833968582c67f55cc59dcc6c8d3fe5",
  "seeders": 1,
  "completed": 0,
  "leechers": 0,
  "peers": [
    {
      "peer_id": {
        "id": "2d7142343431302d7358376d33786d2877674179",
        "client": "qBittorrent"
      },
      "peer_addr": "192.168.1.88:17548",
      "updated_milliseconds_ago": 385,
      "uploaded": 0,
      "downloaded": 0,
      "left": 0,
      "event": "None"
    }
  ]
}

I generally like to add the units in the attribute name.

We can use any other name like: not_updated_duration_in_msecs, not_updated_duration_in_msecs, ...

Originally posted by @josecelano in #60 (comment)

[josecelano]

It seems the backend does not use it:

https://github.com/torrust/torrust-index-backend/blob/8fbbb5ba3a80d1206e56a82391cc49740e803f54/src/tracker.rs#L22

so it would be safe to rename it, at least for other projects in the organization. It's a breaking change for external projects using only this tracker.