Add a cross-version-testing crate for libsignal-protocol

By importing past tags of libsignal-protocol, we can check how the
current implementation behaves against previous versions. This initial
test only does v3 (pre-Kyber) 1:1 sessions, but we can add more tests
in the future.

rust/protocol/cross-version-testing/src/lib.rs

@@ -0,0 +1,25 @@
+//
+// Copyright 2023 Signal Messenger, LLC.
+// SPDX-License-Identifier: AGPL-3.0-only
+//
+
+#![allow(clippy::new_without_default)]
+
+pub use libsignal_protocol_current::{CiphertextMessageType, PreKeyBundle};
+
+pub trait LibSignalProtocolStore {
+    fn version(&self) -> &'static str;
+    fn create_pre_key_bundle(&mut self) -> PreKeyBundle;
+    fn process_pre_key_bundle(&mut self, remote: &str, pre_key_bundle: PreKeyBundle);
+    fn encrypt(&mut self, remote: &str, msg: &[u8]) -> (Vec<u8>, CiphertextMessageType);
+    fn decrypt(&mut self, remote: &str, msg: &[u8], msg_type: CiphertextMessageType) -> Vec<u8>;
+}
+
+mod current;
+pub use current::LibSignalProtocolCurrent;
+
+mod v21;
+pub use v21::LibSignalProtocolV21;
+
+mod v12;
+pub use v12::LibSignalProtocolV12;