[bdk_chain_redesign] chain_oracle::Cache

Introduce `chain_oracle::Cache` which is a cache for requests to the
chain oracle. `ChainOracle` has also been moved to the `chain_oracle`
module.

Introduce `get_tip_in_best_chain` method to the `ChainOracle` trait.
This allows for guaranteeing that chain state can be consistent across
operations with `IndexedTxGraph`.

crates/chain/src/local_chain.rs

@@ -22,6 +22,14 @@ pub struct LocalChain {
 impl ChainOracle for LocalChain {
     type Error = Infallible;
 
+    fn get_tip_in_best_chain(&self) -> Result<Option<BlockId>, Self::Error> {
+        Ok(self
+            .blocks
+            .iter()
+            .last()
+            .map(|(&height, &hash)| BlockId { height, hash }))
+    }
+
     fn get_block_in_best_chain(&self, height: u32) -> Result<Option<BlockHash>, Self::Error> {
         Ok(self.blocks.get(&height).cloned())
     }
@@ -153,7 +161,7 @@ impl Deref for ChangeSet {
     }
 }
 
-/// Represents an update failure of [`LocalChain`].j
+/// Represents an update failure of [`LocalChain`].
 #[derive(Clone, Debug, PartialEq)]
 pub enum UpdateError {
     /// The update cannot be applied to the chain because the chain suffix it represents did not