Call Aggregator API to verify Signer registration

[dlachaume]

Content

This PR includes an update of verify_signer_registration script.
It now calls /epoch-settings and/signers/registered/ to find out whether or not the signer is registered at the current epoch.

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

Comments

Run a Mithril signer as an SPO guide was updated for both versions (current & next).

Issue(s)

Closes #1152