Enhance machine readable logs in client

[dlachaume]

Content

This PR includes an update of mithril-client-cli.

• Fix the case inconsistency issue (use only snake case now).
• Fix a formatting problem with seconds_left and seconds_elapsed ("5s" and "001ms" was formatted as "5.1" in logs, instead of "5.001").
• Add timestamp in JSON progress logs.
• Add --log-format-json argument in mithril-client-cli commands, which displays slog logs in json format.
• Add --log-output argument in mithril-client-cli commands to redirect the slog logs to a file.

Important

This PR includes impacting modifications mithril-client-cli. JSON progress logs were modified:

• bytesDownloaded is now bytes_downloaded
• bytesTotal is now bytes_total
• secondsLeft is now seconds_left
• secondsElapsed is now seconds_elapsed

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)

Closes #1234

[github-actions]

Test Results

    3 files  ±  0    32 suites  ±0   7m 36s ⏱️ -28s
739 tests +  4  739 ✔️ +  4  0 💤 ±0  0 ❌ ±0 
816 runs  +12  816 ✔️ +12  0 💤 ±0  0 ❌ ±0 

Results for commit 9f17b50. ± Comparison against base commit b7e4f13.

♻️ This comment has been updated with latest results.

[jpraynaud]

LGTM 👍