Feat(e2e): support multiple aggregators in the e2e tests

[jpraynaud]

Content

This PR includes the adaptation of the e2e tests to support multiple aggregators:

• The relay has been updated to support both Passthrough (messages are sent to the configured aggregator endpoint) and P2P (messages are sent to the P2P network) modes for both the signer registration and signature registration. The configuration options have been updated in that sense
• The end to end test configuration has evolved:
  • number_of_aggregators and number_of_signers are specified instead of number_of_pool_nodes
  • use_p2pmode has been replaced by more appropriate use_relays
  • relay_signer_registration_mode and relay-signature_registration_mode have been added (used with the use_relays option)
• RunOnly mode of the e2e test has been adapted to support concurrently multiple aggregators
• Spec mode of the e2e test has been adapted to support concurrently multiple aggregators
• Slave registration of the aggregator has been fixed as it was not targeting the correct verification keys. The associated integration test has been rewritten for finer testing of evolving Mithril stake distribution for each epoch.

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
  • All check jobs of the CI have succeeded
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested
• Documentation
  • No new TODOs introduced

Issue(s)

Closes #2361

[github-actions]

Test Results

    3 files  ±0     57 suites  ±0   11m 49s ⏱️ +14s
1 774 tests +2  1 774 ✅ +2  0 💤 ±0  0 ❌ ±0 
2 172 runs  +2  2 172 ✅ +2  0 💤 ±0  0 ❌ ±0 

Results for commit 2ff1fe1. ± Comparison against base commit 81e90f7.

♻️ This comment has been updated with latest results.