babe: support online configuration upgrades (#5514)

* babe: support online configuration upgrades

* Switch to use NextConfigDescriptor instead of changing runtime interface

* Fix tests

* epoch-changes: map function that allows converting with different epoch types

* Add migration script for the epoch config change

* Fix migration tests

* Fix migration: Epoch should be EpochV0

* Update client/consensus/babe/src/lib.rs

Co-Authored-By: André Silva <123550+andresilva@users.noreply.github.com>

* Fix new epochChanges version

* Fix unused imports

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

substrate/client/consensus/babe/src/migration.rs

@@ -0,0 +1,64 @@
+use codec::{Encode, Decode};
+use sc_consensus_epochs::Epoch as EpochT;
+use crate::{
+	Epoch, SlotNumber, AuthorityId, BabeAuthorityWeight, BabeGenesisConfiguration,
+	BabeEpochConfiguration, VRF_OUTPUT_LENGTH, NextEpochDescriptor,
+};
+
+/// BABE epoch information, version 0.
+#[derive(Decode, Encode, PartialEq, Eq, Clone, Debug)]
+pub struct EpochV0 {
+	/// The epoch index.
+	pub epoch_index: u64,
+	/// The starting slot of the epoch.
+	pub start_slot: SlotNumber,
+	/// The duration of this epoch.
+	pub duration: SlotNumber,
+	/// The authorities and their weights.
+	pub authorities: Vec<(AuthorityId, BabeAuthorityWeight)>,
+	/// Randomness for this epoch.
+	pub randomness: [u8; VRF_OUTPUT_LENGTH],
+}
+
+impl EpochT for EpochV0 {
+	type NextEpochDescriptor = NextEpochDescriptor;
+	type SlotNumber = SlotNumber;
+
+	fn increment(
+		&self,
+		descriptor: NextEpochDescriptor
+	) -> EpochV0 {
+		EpochV0 {
+			epoch_index: self.epoch_index + 1,
+			start_slot: self.start_slot + self.duration,
+			duration: self.duration,
+			authorities: descriptor.authorities,
+			randomness: descriptor.randomness,
+		}
+	}
+
+	fn start_slot(&self) -> SlotNumber {
+		self.start_slot
+	}
+
+	fn end_slot(&self) -> SlotNumber {
+		self.start_slot + self.duration
+	}
+}
+
+impl EpochV0 {
+	/// Migrate the sturct to current epoch version.
+	pub fn migrate(self, config: &BabeGenesisConfiguration) -> Epoch {
+		Epoch {
+			epoch_index: self.epoch_index,
+			start_slot: self.start_slot,
+			duration: self.duration,
+			authorities: self.authorities,
+			randomness: self.randomness,
+			config: BabeEpochConfiguration {
+				c: config.c,
+				secondary_slots: config.secondary_slots,
+			},
+		}
+	}
+}