feat: the configuration can be serialized as JSON

We will print to logs the final configuration used to run the tracker (after Figment processes
all sources):

```output
Loading extra configuration from file: `storage/tracker/etc/tracker.toml` ...
2024-07-01T15:29:09.785334Z  INFO torrust_tracker::bootstrap::logging: Logging initialized
2024-07-01T15:29:09.785862Z  INFO torrust_tracker::bootstrap::app: Configuration:
 {
  "logging": {
    "log_level": "info"
  },
  "core": {
    "announce_policy": {
      "interval": 120,
      "interval_min": 120
    },
    "database": {
      "driver": "Sqlite3",
      "path": "./storage/tracker/lib/database/sqlite3.db"
    },
    "inactive_peer_cleanup_interval": 600,
    "listed": false,
    "net": {
      "external_ip": "0.0.0.0",
      "on_reverse_proxy": false
    },
    "private": true,
    "tracker_policy": {
      "max_peer_timeout": 900,
      "persistent_torrent_completed_stat": false,
      "remove_peerless_torrents": true
    },
    "tracker_usage_statistics": true
  },
  "udp_trackers": null,
  "http_trackers": null,
  "http_api": null,
  "health_check_api": {
    "bind_address": "127.0.0.1:1313"
  }
}
2024-07-01T15:29:09.785879Z  WARN torrust_tracker::app: No services enabled in configuration
2024-07-01T15:29:09.785920Z  INFO torrust_tracker::app: No UDP blocks in configuration
2024-07-01T15:29:09.785923Z  INFO torrust_tracker::app: No HTTP blocks in configuration
2024-07-01T15:29:09.785924Z  INFO torrust_tracker::app: No API block in configuration
2024-07-01T15:29:09.785941Z  INFO HEALTH CHECK API: Starting on: http://127.0.0.1:1313
2024-07-01T15:29:09.786035Z  INFO HEALTH CHECK API: Started on: http://127.0.0.1:1313
```

Author: josecelano

Cargo.lock

@@ -19,6 +19,7 @@ camino = { version = "1.1.6", features = ["serde", "serde1"] }
 derive_more = "0"
 figment = { version = "0.10.18", features = ["env", "test", "toml"] }
 serde = { version = "1", features = ["derive"] }
+serde_json = { version = "1", features = ["preserve_order"] }
 serde_with = "3"
 thiserror = "1"
 toml = "0"