Batch aggregation and broadcast

[corverroos]

🎯 Problem to be solved

Currently, the core workflow batches all duties in the same the slot all the way from Scheduler to ParSigEx. But due to Aggregator requiring a different API, neither ParSignedDutySet, not SignedDutySet, we opted for non-batched individual API for aggregation and therefore broadcasting.

Batching broadcasts should have a positive effect on performance, both in charon broadcast delay as well as beacon node load. This should espeically help in performance clusters.

🛠️ Proposed solution

Change the core workflow API of SigAgg and all subsequent components to something like:

// SigAgg aggregates threshold partial signatures.
type SigAgg interface {
	// Aggregate aggregates the partially signed duties for the DV.
	Aggregate(context.Context, Duty, map[PubKey][]ParSignedData) error

	// Subscribe registers a callback for aggregated signed duties.
	Subscribe(func(context.Context, Duty, SignedDataSet) error)
}