Sign Cardano transactions with ChainPoint based beacon

[jpraynaud]

Why

We want to use a new beacon based on a chain point instead of an immutable file number when signing Cardano transactions.

What

Migrate the CardanoTransactions signed entity type to use a ChainPoint instead of ImmutableFileNumber. A new associated open message is created at every n blocks (value of n needs to be a configuration parameter to be defined) with a fixed offset k' (value of k' needs to be a configuration parameter to be defined which represents the unofficial security parameter).

How

• Update the CardanoTransactions signed entity type to use a ChainPoint internally
• Update the TimePoint to have a ChainPoint or BlockNumber entry that represents the tip in the state machine of the aggregator
• Compute the signed entity type with block_number = ((tip.block_number - k') / n) * n
• Make k' and n parameters configurable in the aggregator with default values: k'=3000 and n=100 (values can be adjusted)
• Make BlockScanner of immutable files compute the upper bound on its own (try to get the latest immutable file number?)
• Clean existing conversion between ImmutableFileNumber and BlockNumber in the database repository
• Adapt signature of CardanoTransactions to the new beacon
• Adapt the CardanoTransactionSnapshot artifact to support the new beacon
• Clean the artifacts for the Cardano transactions in the aggregator database with a new migration