UDP Tracker client: Add option to show response in pretty JSON

[josecelano]

Parent issue: #669
Relates to: #1562

I usually pipe the client output through the console command "jq" to format the JSON response into a more readable format. It would be nice to add an option to do it without requiring an external command:

cargo run -p torrust-tracker-client --bin udp_tracker_client announce udp://tracker.torrust-demo.com:6969/announce 000620bbc6c52d5a96d98f6c0f1dfa523a40df82 | jq

{
  "AnnounceIpv4": {
    "transaction_id": -888840697,
    "announce_interval": 300,
    "leechers": 2,
    "seeders": 7,
    "peers": [
      "2.137.108.79:34688",
      "37.109.147.11:8443",
      "86.42.165.70:8999",
      "93.159.28.2:51413",
      "185.72.186.215:51413",
      "185.164.140.189:7990",
      "188.146.16.204:6881",
      "195.117.101.49:49512"
    ]
  }
}

The new command option could be format:

cargo run -p torrust-tracker-client --bin udp_tracker_client announce udp://tracker.torrust-demo.com:6969/announce 000620bbc6c52d5a96d98f6c0f1dfa523a40df82 --format pretty