[TASK] Store historical (round_id, public key, public_key_signature) before killing the storage in pallet-dkg-metadata

[drewstone]

Overview

Adding a new historical storage item to the pallet. We will need to take care that we represent the data correctly and the key signing said data. Recall that a refresh proposal is Sign(concat(round_id, next_key), signing_key).

pub type HistoricalKeyStorage<T: Config> = StorageDoubleMap<_, Blake2_256, (RoundId), (PreviousKey, NextKey, Signature)>;