Add generic threshold aggregation functionality to DKG

[corverroos]

Problem to be solved

At this point, DKG supports distributed aggregation of lock hash partial signatures.
There are however additional aggregation signatures that should be created as part of the DKG command.

This includes:

• Threshold aggregate signature of deposit data message roots (one for each validator).
• Maybe other things...?

The current signing logic:

• is actually broken, since all peers should complete wiring BEFORE any peer sends a message (otherwise those messages are dropped).
• only supports single lock hash signing and aggregation by all validators.

Proposed solution

• Refactor DKG to support "general distributed aggregation of partial signatures".
• Do this by extracting a aggregator type/function.
• It should be wired and constructed before any peers send signing messages (before switch def.DKGAlgorithm step).
• It should provide a function Aggregate(core.ParSignedDataSet) (map[core.PubKey]core.AggSignedData, error)
• It should use the aggSignLockHash logic at its core (including parsigdb and parsigex and a custom sigAgg function).
• This Aggregate function will then be called after creating a core.ParSignedDataSet for the lock hash (same hash for all pubkeys)

Out of Scope

• This Aggregate function will later be after creating a core.ParSignedDataSet for the deposit datas hash (different hashes for each pubkey).