Remove certificate creation responsability from the multi-signer

[Alenar]

Content

This PR change the behavior of the aggregator by transferring the certificate creation from the multi-signer to the state machine.

Pre-submit checklist

• Branch
  • Tests are provided (if possible)
  • 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)

Fix #638 that's part of #398

[github-actions]

Test Results

    3 files  ±0    28 suites  ±0   2m 36s ⏱️ -30s
341 tests +2  341 ✔️ +2  0 💤 ±0  0 ❌ ±0 
394 runs  +2  394 ✔️ +2  0 💤 ±0  0 ❌ ±0 

Results for commit a089b9d. ± Comparison against base commit e18920a.

This pull request removes 1 and adds 3 tests. Note that renamed tests count towards both.

runtime::runner::tests ‑ test_create_and_save_certificate_ko_epoch_gap

certificate_creator::tests ‑ test
runtime::runner::tests ‑ test_create_new_working_certificate
runtime::runner::tests ‑ test_create_new_working_certificate_ko_epoch_gap

♻️ This comment has been updated with latest results.

[jpraynaud]

LGTM 👍