Support optional `cardano_transactions_signing_config`

[jpraynaud]

Why

There exist a strong requirement to use a CardanoTransactionsSigningConfig in the SignedEntityConfig type. In the future, when the CardanoTransaction signed entity type will be sunset and this option will not be available anymore. This would lead the previous versions of the signer to panic and stop working. Thus, we want to refactor the signer to support an optional CardanoTransactionsSigningConfig.

What

Support an optional CardanoTransactionsSigningConfig in the signer.

How

• Make the cardano_transactions_signing_config field of SignedEntityConfig optional
• Return an error in the time_point_to_signed_entity function if the cardano_transactions_signing_config is not available
• Update the signer to support the optional cardano_transactions_signing_config
• Update the aggregator to support the optional cardano_transactions_signing_config
• Support migration of the EpochSettingsRecord in the aggregator