Update SetDecided to include the validators

Necessary for light-client protocols to follow along with consensus. Arguably,
anyone handling GRANDPA's consensus could peek at this through the consensus
commit anyways, but we shouldn't so defer that.

substrate/abi/src/validator_sets.rs

@@ -1,3 +1,5 @@
+use alloc::vec::Vec;
+
 use borsh::{BorshSerialize, BorshDeserialize};
 
 use serai_primitives::{
@@ -96,6 +98,8 @@ pub enum Event {
   SetDecided {
     /// The set decided.
     set: ValidatorSet,
+    /// The validators decided to be included in the set.
+    validators: Vec<(SeraiAddress, KeyShares)>,
   },
   /// A validator set has set their keys.
   SetKeys {