Feat: add Cardano era retrieval in chain observer implementations

[dlachaume]

Content

This PR includes the retrieval of the current Cardano era in both the CardanoCliChainObserver and PallasChainObserver.

Pre-submit checklist

• Branch
  • [x] Tests are provided (if possible)
  • [x] Crates versions are updated (if relevant)
  • [x] Commit sequence broadly makes sense
  • [x] Key commits have useful messages
• PR
  • [x] No clippy warnings in the CI
  • [x] Self-reviewed the diff
  • [x] Useful pull request description
  • [x] Reviewer requested

Issue(s)

Relates to #2071

[github-actions[bot]]

Test Results

    4 files  ±0     51 suites  ±0   10m 25s ⏱️ -12s 1 417 tests +2  1 417 ✅ +2  0 💤 ±0  0 ❌ ±0  1 628 runs  +2  1 628 ✅ +2  0 💤 ±0  0 ❌ ±0 

Results for commit 73d1fd4f. ± Comparison against base commit 6f6aed3b.

:recycle: This comment has been updated with latest results.