Refactor core workflow types

[corverroos]

Problem to solve

The core workflow types (DutyArg, DutyArgSet, etc) can be simplified.

Also, I think we should refactor DV identifier from VIdx PubKey. This was the original design, and since we now have explicit DutyArg/FetchArg type, we do not need to pull query args to the top level of the type system. Another reason why DVs should be identified by the pubkey rather than then VIdx, is that when downstream components need to lookup TSS from the manifest, doing so via the VIdx require additional shared state, while PubKey doesn't.

Proposed solution

If we assume the project structure in #155:

• Refactor VIdx -> PubKey
• Keep Duty
• Rename DutyArg -> FetchArg (and FetchArgSet)
• Rename DutyData -> UnsignedData (and UnsignedDataSet)
• Rename SignedDutyData -> ParSignedData (and ParSignedDataSet)
• Introduce -> AggSignedData (and AggSignedDataSet)
• Rename SigDB -> ParSigDB
• Rename SigEx -> ParSigEx
• Rename AggDB -> AggSigDB