Remove `network` field from `CardanoDbBeacon`

[jpraynaud]

Why

The CardanoDbBeacon holds a network field which should be removed as this information is not necessary to operate the Mithril network.

What

Remove the network field in the CardanoDbBeacon without breaking-change.

How

• Ensure retro-compatibility between nodes in http messages:
  • Add a CardanoDbBeaconMessagePart with a optional network field
  • Use this new beacon, and adapt dependent code, in:
    • Certificate and CertificateList messages
    • CertificatePending message
    • RegisterSignature message
    • Snapshot, SnapshotList, and SnapshotDownload message
• Adapt the computation of the hash in the compute_hash of the CardanoImmutableDigester to retrieve the network value:
  • from the configuration in the signer
  • from the configuration in the aggregator
  • from the certificate metadata in the client
  • Golden test the computation of the digest to make sure there is no breaking change
• Remove the network field in the CardanoDbBeacon
• ⚠️ Recompute certificate chain to be scheduled with next distribution