Refactor `AggregatorClient` trait in signer

[jpraynaud]

Why

The AggregatorClient trait has a confusing name and may have too much responsibility. We want to refactor it and split it in multiple traits.

What

Refactor the AggregatorClient trait in multiple simple traits.

How

• Remove the register_signature function and use directly the publish function from the SignaturePublisher
• Implement the SignaturePublisher on the internal aggregator client
• Move the register_signer function to a new SignerRegistrationPublisher trait
• Implement the SignerRegistrationPublisher on the internal aggregator client
• Remove the retrieve_aggregator_features which is not used in production test
• Rename the retrieve_epoch_settings function to retrieve_all_signer_registrations in a new SignersRegistrationRetriever (name to be refined)
• Refactor the integration test extensions which use retrieve_aggregator_features and retrieve_epoch_settings (and rename certificate_handler to a better name)