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 <[email protected]>

* Fix new epochChanges version

* Fix unused imports

Co-authored-by: André Silva <[email protected]>