Implement transactions verification in client library

[jpraynaud]

Why

The client library needs to provide a way to generate, verify and certify proofs for a list of transactions.

What

Implement new API function for verifying a list of proofs generated by the aggregator in the client library.

How

• Implement transaction verification function in library:
  • Call aggregator to generate a list of proofs for the transaction list
  • Return the list of certified transactions (and list of not certified?)
  • Verify the proofs for all the certified transactions
  • Retrieve the latest artifact (and its certificate hash)
  • Verify that the Merkle root of the Merkle proofs is signed by the certificate
  • Verify that the certificate chain is valid
• Implement a function that lists artifacts or get latest produced artifact?
• Update Mithril events feedback mechanism
• Update documentation
• Add feature flag unstable?

Later

• Add an example in the repository (later, will be done in another PR)