refactor: separate interface and service in bls_aggr

[damiramirez]

What Changed?

This PR refactors the BLS Aggregator by separating its interface from the internal service logic.

• Add new struct ServiceHandle to serve as the interface for communicating with the BLS Aggregator service. This interface provides two new methods
  • initialize_task: Sends a message to the BLS Aggregator Service to initialize a new task.
  • process_signature: Sends a message to the BLS Aggregator Service to process a signature.
• Add new struct AggregateReceiver to receive the aggregated responses from the BLS Aggregator Service.
• Remove channels from BlsAggregatorService
• Add new methods to BlsAggregatorService:
  • start: Starts the BLS Aggregator Service running the main loop in background. This method return a tuple with ServiceHandle and AggregateReceiver. User can use these methods to interact with the service.
  • run: Runs the main loop of the BLS Aggregator Service.
• Remove initialize_new_task and process_new_signature functions since their logic is now integrated in run().

Close #257

Reviewer Checklist

• New features are tested and documented
• PR updates the changelog with a description of changes
• PR has one of the changelog-X labels (if applies)
• Code deprecates any old functionality before removing it

[MegaRedHand]

Great work!