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/authorship.rs

@@ -19,7 +19,7 @@
 use merlin::Transcript;
 use sp_consensus_babe::{
 	AuthorityId, BabeAuthorityWeight, BABE_ENGINE_ID, BABE_VRF_PREFIX,
-	SlotNumber, AuthorityPair, BabeConfiguration
+	SlotNumber, AuthorityPair,
 };
 use sp_consensus_babe::digests::{PreDigest, PrimaryPreDigest, SecondaryPreDigest};
 use sp_consensus_vrf::schnorrkel::{VRFOutput, VRFProof};
@@ -147,12 +147,11 @@ fn claim_secondary_slot(
 pub fn claim_slot(
 	slot_number: SlotNumber,
 	epoch: &Epoch,
-	config: &BabeConfiguration,
 	keystore: &KeyStorePtr,
 ) -> Option<(PreDigest, AuthorityPair)> {
-	claim_primary_slot(slot_number, epoch, config.c, keystore)
+	claim_primary_slot(slot_number, epoch, epoch.config.c, keystore)
 		.or_else(|| {
-			if config.secondary_slots {
+			if epoch.config.secondary_slots {
 				claim_secondary_slot(
 					slot_number,
 					&epoch.authorities,