Further flesh out tributary scanning

Renames `label` to `round` since `Label` was renamed to `SigningProtocolRound`.

Adds some more context-less validation to transactions which used to be done
within the custom decode function which was simplified via the usage of borsh.

Documents in processor-messages where the Coordinator sends each of its
messages.

substrate/validator-sets/primitives/src/lib.rs

@@ -114,8 +114,8 @@ pub struct Slash {
       deserialize_with = "serai_primitives::borsh_deserialize_public"
     )
   )]
-  key: Public,
-  points: u32,
+  pub key: Public,
+  pub points: u32,
 }
 #[derive(Clone, PartialEq, Eq, Debug, Encode, Decode, TypeInfo, MaxEncodedLen)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]