Implement signable builder for Incremental Cardano DB

[jpraynaud]

Why

In order to certify the Cardano database and provide differential restoration, we need to implement the signable builders for the CardanoDatabase signed entity type.

What

Implemement the signable builder for the CardanoDatabase signed entity type.

How

• Activate optionally the CardanoDatabase signed entity type in the aggregator
• Adjust the timeout on the CardanoDatabase signed entity type to 1800s
• Implement the signable builder:
  • Create a Merkle tree with leaves (immutable_digest) (same computation for the digest as in CardanoImmutableFilesFull)
  • Use the Merkle root as the message to sign
• Create a boilerplate artifact builder?
• Activate the CardanoDatabase signed entity type in the e2e test