Fix: ensure epoch consistency during epoch changes when signer is signing

[dlachaume]

Content

This PR includes a fix that was triggered by flakiness failures on the CI.
The problem occurred when there was an epoch change after the signer had already begun its signature process.
To resolve this, the current epoch is now passed as a parameter when the signer retrieves the beacon to sign, instead of retrieving it with the ticker service, ensuring consistency throughout the signature process.

This fixes all the CI errors such as:
Mithril End to End test in failed: Minimum expected mithril stake distribution epoch not reached : 18 < 20

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
• Documentation
  • Update README file (if relevant)
  • Update documentation website (if relevant)
  • Add dev blog post (if relevant)

Issue(s)

Closes #2044

[github-actions]

Test Results

    4 files  ±0     55 suites  ±0   10m 32s ⏱️ +2s
1 395 tests ±0  1 395 ✅ ±0  0 💤 ±0  0 ❌ ±0 
1 609 runs  ±0  1 609 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 2c18bec. ± Comparison against base commit 38b3203.

♻️ This comment has been updated with latest results.

[jpraynaud]

LGTM 👍

[sfauvel]

LGTM