[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/lib.rs

@@ -31,6 +31,8 @@ pub mod sparse_chain;
 mod tx_data_traits;
 pub mod tx_graph;
 pub use tx_data_traits::*;
+mod chain_oracle;
+pub use chain_oracle::*;
 
 #[doc(hidden)]
 pub mod example_utils;