Implement a signature processor for DMQ node in aggregator

[jpraynaud]

Why

We need to develop a signature processor that continuously receives signatures from the DMQ node and registers them with the aggregator's certifier service. This processor should operate persistently on a separate thread within the node.

What

Implement a signature processor in the aggregator.

How

• Add a SignatureConsumer trait in the aggregator
• Implement a noop for the SignatureConsumer trait
• Add a SignatureProcessor trait in the aggregator
• Implement a SequentialSignatureProcessor that continuously receives signatures from a SignaturePublisher and registers them with the certifier service
• Run the SignatureProcessor in a separate thread of the aggregator serve command