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/system.rs

@@ -0,0 +1,8 @@
+use frame_support::dispatch::{DispatchInfo, DispatchError};
+
+#[derive(Clone, PartialEq, Eq, Debug, scale::Encode, scale::Decode)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+pub enum Event {
+  ExtrinsicSuccess { dispatch_info: DispatchInfo },
+  ExtrinsicFailed { dispatch_error: DispatchError, dispatch_info: DispatchInfo },
+}