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

• [ ] 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
• [ ] :warning: Recompute certificate chain to be scheduled with next distribution