[examples_redesign] Introduce example_cli package

This is the equivalent of `keychain_tracker_example_cli` that works with
the redesigned structures.

crates/chain/src/chain_oracle.rs

@@ -19,4 +19,7 @@ pub trait ChainOracle {
         block: BlockId,
         chain_tip: BlockId,
     ) -> Result<Option<bool>, Self::Error>;
+
+    /// Get the best chain's chain tip.
+    fn get_chain_tip(&self) -> Result<Option<BlockId>, Self::Error>;
 }