Return babe configuration information in the babe api epoch functions (#8072)

* Make changes

* Add serialize/deserialize, copy babe epoch config defaults from node runtime

* Fix line widths and turn default features off for serde

* Remove ser/deser from Epoch, fix node-cli

* Apply suggestions

* Add comment to BABE_GENESIS_EPOCH_CONFIG in bin

* Apply suggestions

* Add a sketchy migration function

* Add a migration test

* Check for PendingEpochConfigChange as well

* Make epoch_config in node-cli

* Move updating EpochConfig out of the if

* Fix executor tests

* Calculate weight for add_epoch_configurations

* Fix babe test

* Apply suggestions from code review

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

* Add more asserts to tests, remove unused changes to primitives/slots

* Allow setting the migration pallet prefix

* Rename to BabePalletPrefix

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

substrate/bin/node/runtime/src/lib.rs

@@ -119,6 +119,13 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	transaction_version: 2,
 };
 
+/// The BABE epoch configuration at genesis.
+pub const BABE_GENESIS_EPOCH_CONFIG: sp_consensus_babe::BabeEpochConfiguration =
+	sp_consensus_babe::BabeEpochConfiguration {
+		c: PRIMARY_PROBABILITY,
+		allowed_slots: sp_consensus_babe::AllowedSlots::PrimaryAndSecondaryPlainSlots
+	};
+
 /// Native version.
 #[cfg(any(feature = "std", test))]
 pub fn native_version() -> NativeVersion {
@@ -1274,10 +1281,10 @@ impl_runtime_apis! {
 			sp_consensus_babe::BabeGenesisConfiguration {
 				slot_duration: Babe::slot_duration(),
 				epoch_length: EpochDuration::get(),
-				c: PRIMARY_PROBABILITY,
+				c: BABE_GENESIS_EPOCH_CONFIG.c,
 				genesis_authorities: Babe::authorities(),
 				randomness: Babe::randomness(),
-				allowed_slots: sp_consensus_babe::AllowedSlots::PrimaryAndSecondaryPlainSlots,
+				allowed_slots: BABE_GENESIS_EPOCH_CONFIG.allowed_slots,
 			}
 		}