Implement configurable Chain Observer in aggregator

[jpraynaud]

Content

This PR includes a new Chain Observer builder in the aggregator and the signer that uses a configuration parameter to determine which chain observer to create. This allows to fix the aggregator stress test that imperatively needs to rely on the Cardano Cli chain observer while using the Pallas chain observer by default.

Chain observer types:

Use case	Chain Observer Type
Mithril networks infrastructure	pallas
End to end tests	pallas
Aggregator stress tests	cardano-cli

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
• Documentation
  • Update documentation website (if relevant)

Issue(s)

Closes #1396

[github-actions]

Test Results

  3 files  ±0   32 suites  ±0   8m 45s ⏱️ +31s
739 tests ±0  739 ✅ ±0  0 💤 ±0  0 ❌ ±0 
812 runs  ±0  812 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 0fb579e. ± Comparison against base commit 673a87d.

♻️ This comment has been updated with latest results.

[dlachaume]

LGTM 👍