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/babe.rs

@@ -0,0 +1,15 @@
+use sp_consensus_babe::EquivocationProof;
+
+use serai_primitives::Header;
+
+#[derive(Clone, PartialEq, Eq, Debug, scale::Encode, scale::Decode)]
+pub struct ReportEquivocation {
+  pub equivocation_proof: Box<EquivocationProof<Header>>,
+  pub key_owner_proof: (),
+}
+
+#[derive(Clone, PartialEq, Eq, Debug, scale::Encode, scale::Decode)]
+pub enum Call {
+  report_equivocation(ReportEquivocation),
+  report_equivocation_unsigned(ReportEquivocation),
+}