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/grandpa.rs

@@ -0,0 +1,24 @@
+use sp_consensus_grandpa::EquivocationProof;
+
+use serai_primitives::{BlockNumber, SeraiAddress};
+
+#[derive(Clone, PartialEq, Eq, Debug, scale::Encode, scale::Decode)]
+pub struct ReportEquivocation {
+  pub equivocation_proof: Box<EquivocationProof<[u8; 32], BlockNumber>>,
+  pub key_owner_proof: (),
+}
+
+#[derive(Clone, PartialEq, Eq, Debug, scale::Encode, scale::Decode)]
+pub enum Call {
+  report_equivocation(ReportEquivocation),
+  report_equivocation_unsigned(ReportEquivocation),
+}
+
+#[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 {
+  NewAuthorities { authority_set: Vec<(SeraiAddress, u64)> },
+  Paused,
+  Resumed,
+}