Minotaur-Mithril PoC follow-up

## Why
The Minotaur project team has worked on a prototype of implementation of the Mithril client in their developments.
Their need is to certify UTxOs from some pre-specified address. In order to do so, they have used the existing Cardano Transaction certification. The current implementation was fine for a Proof of Concept, but some new features would be needed in order to be ready for production use.

## What

New features needed for production usage:
- Enhanced certification of the transactions with combination of `(tx_hash, block_hash, block_number)` instead of only the `tx_hash`.
- Certification as close as possible from the tip of the chain (and provide finality probability depending on depth from tip of the chain when snapshot is created).
- Provide a way to download the certificate chain with the client library.
- Provide a way to verify a certificate chain previously downloaded with the client library.