BabeApi_current_epoch() and BabeApi_next_epoch() don't give any information about configuration

[tomaka]

The Babe runtime module has the current_epoch() and next_epoch() functions that return the current and next epoch.
However these functions don't give any information about the configuration, which can potentially change between epochs (even though it never does in practice at the moment).

The configuration should either be added to the returned struct, or added a separate methods, or something like this.

Would be useful for GrandPa warp sync, in order to know about a configuration update that is pending at moment of the finalized block we sync to.

[tomaka]

Done in #8072