Add era command in aggregator cli

[jpraynaud]

Content

This PR includes a Era command in the aggregator cli that:

• Lists the supported eras
• Generate a TxDatum JSON that can be used in an on-chain transaction

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 #755

[github-actions]

Test Results

    3 files  ±0    28 suites  ±0   5m 2s ⏱️ -43s
441 tests +4  441 ✔️ +4  0 💤 ±0  0 ❌ ±0 
509 runs  +4  509 ✔️ +4  0 💤 ±0  0 ❌ ±0 

Results for commit a325ca7. ± Comparison against base commit 87938ad.

This pull request removes 1 and adds 5 tests. Note that renamed tests count towards both.

era::supported_era::tests ‑ from_bad_str

chain_observer::model::test ‑ test_build_tx_datum
era::supported_era::tests ‑ correct_number_of_eras
tools::era::tests ‑ generate_tx_datum_ok
tools::era::tests ‑ generate_tx_datum_wrong_epochs
tools::era::tests ‑ get_supported_eras_list

♻️ This comment has been updated with latest results.