Create new signed entity type for Incremental Cardano DB

[jpraynaud]

Why

We want to bootstrap the signed entity type for the incremental Cardano database signature.

What

Create the associated signed entity type and the associated database migrations.

How

Important

Do not use unimplemented! or todo! macro with the default implementation, return error instead.

• Create the new CardanoDatabase(CardanoDbBeacon) signed entity type
• Create the associated database migrations in the:
  • signer
  • aggregator