Add an unknown default for Chain type

Change-Id: I1c175c906311063dd10db4ea3ed693098592585d

solana/anchor-bridge/programs/anchor-bridge/src/types.rs

@@ -18,5 +18,12 @@ pub struct BridgeConfig {
 #[repr(u8)]
 #[derive(AnchorSerialize, AnchorDeserialize, Clone, Copy, Debug)]
 pub enum Chain {
+    Unknown,
     Solana = 1u8,
 }
+
+impl Default for Chain {
+    fn default() -> Self {
+        Chain::Unknown
+    }
+}