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.

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