Refactor BLS aggregation service to expose interface via handle

[MegaRedHand]

Currently, we have a lot of mutexes and shared state in the BLS aggregation service. We could simplify this by splitting the struct into service and interface. To do this, we:

1. add a start function or similar, which consumes the service and starts it as a background task, returning a "handle" to it
2. implement a "handle" struct, which stores all channels used to communicate with the service, but no service state
3. move the current interface to the handle

An example interface would be:

#[derive(Debug, Clone)]
pub struct ServiceHandle {
    task_sender: UnboundedSender<TaskMetadata>,
    response_sender: UnboundedSender<TaskResponseInfo>,
}

#[derive(Debug)]
pub struct AggregateReceiver {
    aggregate_recver: UnboundedReceiver<AggregateSignature>,
}

#[derive(Debug)]
pub struct BlsAggregationService { ... }

impl BlsAggregationService {
    pub fn new() -> Self { ... }

    pub fn start(self) -> (ServiceHandle, AggregateReceiver) { ... }
}