Add ABI crate

Call and Event are both from the pallets, which are AGPL licensed. Accordingly,
they make serai-client AGPL licensed when serai-client must end up MIT
licensed. This creates a MIT-licensed variant of Calls and Events such that
they can be used by serai-client, enabling transitioning it to MIT.

Relevant to https://github.com/serai-dex/serai/issues/337.

substrate/abi/src/signals.rs

@@ -0,0 +1,57 @@
+use serai_primitives::{NetworkId, SeraiAddress};
+
+use serai_validator_sets_primitives::ValidatorSet;
+
+pub use serai_signals_primitives as primitives;
+use primitives::SignalId;
+
+#[derive(Clone, PartialEq, Eq, Debug, scale::Encode, scale::Decode)]
+#[cfg_attr(feature = "borsh", derive(borsh::BorshSerialize, borsh::BorshDeserialize))]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+pub enum Call {
+  register_retirement_signal { in_favor_of: [u8; 32] },
+  revoke_retirement_signal { retirement_signal_id: [u8; 32] },
+  favor { signal_id: SignalId, for_network: NetworkId },
+  revoke_favor { signal_id: SignalId, for_network: NetworkId },
+  stand_against { signal_id: SignalId, for_network: NetworkId },
+}
+
+#[derive(Clone, PartialEq, Eq, Debug, scale::Encode, scale::Decode)]
+#[cfg_attr(feature = "borsh", derive(borsh::BorshSerialize, borsh::BorshDeserialize))]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+pub enum Event {
+  RetirementSignalRegistered {
+    signal_id: [u8; 32],
+    in_favor_of: [u8; 32],
+    registrant: SeraiAddress,
+  },
+  RetirementSignalRevoked {
+    signal_id: [u8; 32],
+  },
+  SignalFavored {
+    signal_id: SignalId,
+    by: SeraiAddress,
+    for_network: NetworkId,
+  },
+  SetInFavor {
+    signal_id: SignalId,
+    set: ValidatorSet,
+  },
+  RetirementSignalLockedIn {
+    signal_id: [u8; 32],
+  },
+  SetNoLongerInFavor {
+    signal_id: SignalId,
+    set: ValidatorSet,
+  },
+  FavorRevoked {
+    signal_id: SignalId,
+    by: SeraiAddress,
+    for_network: NetworkId,
+  },
+  AgainstSignal {
+    signal_id: SignalId,
+    who: SeraiAddress,
+    for_network: NetworkId,
+  },
+}