Client snapshot download progress with json output

[Alenar]

Content

This PR enhance mithril-client command snapshot download json output (--json parameter).
Instead of reporting nothing until the command finish using --json will now print:

• The current step of the process, ie:

{"step_num": 1, "total_steps": 7, "message": "Checking local disk info…"}                                                                                                    
{"step_num": 2, "total_steps": 7, "message": "Fetching the certificate's information…"}                                                                                      
{"step_num": 3, "total_steps": 7, "message": "Verifying the certificate chain…"}                                                                                             
{"step_num": 4, "total_steps": 7, "message": "Downloading the snapshot…"}

• The current progress when in downloading (approximately every 333ms), ie:

{ "bytesDownloaded": 2067507114, "bytesTotal": 2438424190, "secondsLeft": 10.408, "secondsElapsed": 58.296 }
{ "bytesDownloaded": 2079647658, "bytesTotal": 2438424190, "secondsLeft": 10.65, "secondsElapsed": 58.629 }
{ "bytesDownloaded": 2091788202, "bytesTotal": 2438424190, "secondsLeft": 9.723, "secondsElapsed": 58.962 }
{ "bytesDownloaded": 2103928746, "bytesTotal": 2438424190, "secondsLeft": 9.380, "secondsElapsed": 59.296 }
{ "bytesDownloaded": 2116069290, "bytesTotal": 2438424190, "secondsLeft": 9.37, "secondsElapsed": 59.629 }
{ "bytesDownloaded": 2128209834, "bytesTotal": 2438424190, "secondsLeft": 8.694, "secondsElapsed": 59.962 }
{ "bytesDownloaded": 2138122154, "bytesTotal": 2438424190, "secondsLeft": 8.414, "secondsElapsed": 60.295 }
{ "bytesDownloaded": 2147870634, "bytesTotal": 2438424190, "secondsLeft": 8.146, "secondsElapsed": 60.629 }

Pre-submit checklist

• Branch
  • Tests are provided (if possible)
  • Crates versions are updated (if relevant)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
• PR
  • No clippy warnings in the CI
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested
• Documentation
  • Update README file (if relevant)
  • Update documentation website (if relevant)
  • Add dev blog post (if relevant)

Issue(s)

Relates to #1095

[dlachaume]

LGTM 🚀

[github-actions]

Test Results

    3 files  ±0    17 suites  ±0   6m 13s ⏱️ +3s
671 tests ±0  671 ✔️ ±0  0 💤 ±0  0 ❌ ±0 
727 runs  ±0  727 ✔️ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit e9dc557. ± Comparison against base commit bfe3867.