Slash reports (#523)

* report_slashes plumbing in Substrate

Notably delays the SetRetired event until it provides a slash report or the set
after it becomes the set to report its slashes.

* Add dedicated AcceptedHandover event

* Add SlashReport TX to Tributary

* Create SlashReport TXs

* Handle SlashReport TXs

* Add logic to generate a SlashReport to the coordinator

* Route SlashReportSigner into the processor

* Finish routing the SlashReport signing/TX publication

* Add serai feature to processor's serai-client

substrate/abi/src/validator_sets.rs

@@ -1,3 +1,5 @@
+use sp_core::{ConstU32, bounded_vec::BoundedVec};
+
 pub use serai_validator_sets_primitives as primitives;
 
 use serai_primitives::*;
@@ -12,6 +14,11 @@ pub enum Call {
     key_pair: KeyPair,
     signature: Signature,
   },
+  report_slashes {
+    network: NetworkId,
+    slashes: BoundedVec<(SeraiAddress, u32), ConstU32<{ MAX_KEY_SHARES_PER_SET / 3 }>>,
+    signature: Signature,
+  },
   allocate {
     network: NetworkId,
     amount: Amount,
@@ -41,6 +48,12 @@ pub enum Event {
     set: ValidatorSet,
     key_pair: KeyPair,
   },
+  AcceptedHandover {
+    set: ValidatorSet,
+  },
+  SetRetired {
+    set: ValidatorSet,
+  },
   AllocationIncreased {
     validator: SeraiAddress,
     network: NetworkId,
@@ -57,7 +70,4 @@ pub enum Event {
     network: NetworkId,
     session: Session,
   },
-  SetRetired {
-    set: ValidatorSet,
-  },
 }