List immutables resilience

[Alenar]

Content

This PR enhance the immutable files listing, making it more resilient:

• Instead of simply skipping the last three files to remove an incomplete trio, it skip the file(s) with the highest number (meaning that if for example the "secondary" file is missing from the "99" trio this won't remove a file from the "98" trio).
• The condition to be recognize as an immutable is not only being in a "immutable" folder but also to have one of the following file extension: "chunk", "primary", "secondary".
• If will first search for the "immutable" folder and won't walk in its subfolder in search for immutables.
• This point needs to be discussed It will fails if it can't find a "immutable" folder instead of returning an empty list.

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

Issue(s)

Closes #1450

[github-actions]

Test Results

  3 files  ±0   33 suites  ±0   8m 42s ⏱️ -21s
787 tests +3  787 ✅ +8  0 💤 ±0  0 ❌  - 5 
860 runs  +3  860 ✅ +8  0 💤 ±0  0 ❌  - 5 

Results for commit 909936e. ± Comparison against base commit 659da2e.

♻️ This comment has been updated with latest results.

[jpraynaud]

LGTM 👍

I noticed few typos below.
Also I'm OK to fail if no immutable file is found.