Compute message to sign for Cardano transactions

[jpraynaud]

Content

This PR includes:

• Computation of the Merkle tree of the Cardano transactions set and the associated Merkle root
• Use the Merkle root as a protocol message part to be signed in the signer and aggregator

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

Issue(s)

Closes #1436

[github-actions]

Test Results

  3 files  ±0   33 suites  ±0   8m 53s ⏱️ - 1m 37s
784 tests +6  784 ✅ +6  0 💤 ±0  0 ❌ ±0 
857 runs  +6  857 ✅ +6  0 💤 ±0  0 ❌ ±0 

Results for commit 50c9bc7. ± Comparison against base commit c762166.

♻️ This comment has been updated with latest results.