Implement artifact routes in the aggregator

[jpraynaud]

Why

In order to give the capability to a client to certify a specific list of transactions, the aggregator needs to give access to the artifacts produced for the CardanoTransactions signed entity type.

What

Implement new routes in the aggregator which list and detail produced artifacts for the Cardano transactions.

How

• Implement artifact routes in aggregator:
  • Implement route artifact/cardano-transactions that list artifacts
  • Implement route artifact/cardano-transaction/{digest} that gets an artifact
  • Update OpenAPI specs
• Adapt end to end tests to verify that artifacts are produced for Cardano transactions