Store Block Range Merkle roots in signer and aggregator databases

[jpraynaud]

Why

In order to provide faster signature and proof generation of the Cardano transactions, we need to store and use Block range Merkle trees roots in the databases of the signer and aggregator.

What

As soon as transactions of a block range are committed to the store, the corresponding block range root needs to be computed and stored in a new table of the store.

How

• Add a new block_range_root table to the cardano_transactions database to store the new data.
• Add to TransactionImporter trait implementations the capacity to computing/storing block range roots.