Enhance logs in mithril-relay

[Alenar]

Content

This PR refactor the log usage in mithril-relay:

• A dedicated child logger is used for each service
• Each child logger is tagged with its service name in order to identify quickly a log source without the need of a prefix in the log message.
• Capitalize the first letters of all "functional" logs.
• Remove slog_scope to enforce the usage of a name tagged child logger.
• Move away from the log message into a property huge structure (ie: some logs joined errors into their message, those errors have been moved to a "error" property).

Pre-submit checklist

• Branch
  • Tests are provided (if possible)
  • Crates versions are updated (if relevant)
  • CHANGELOG file is 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

Issue(s)

Closes #1981

[github-actions]

Test Results

    4 files  ±0     55 suites  ±0   10m 24s ⏱️ +5s
1 370 tests ±0  1 370 ✅ ±0  0 💤 ±0  0 ❌ ±0 
1 584 runs  ±0  1 584 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit a879a96. ± Comparison against base commit 495f150.

♻️ This comment has been updated with latest results.

[jpraynaud]

LGTM 👍