feat(chain): add get and range methods to CheckPoint

These methods allow us to query for checkpoints contained within the
linked list by height and height range. This is useful to determine
checkpoints to fetch for chain sources without having to refer back to
the `LocalChain`.

Currently this is not implemented efficiently, but in the future, we
will change `CheckPoint` to use a skip list structure.

crates/esplora/tests/blocking_ext.rs

@@ -360,8 +360,8 @@ fn update_local_chain() -> anyhow::Result<()> {
         for height in t.request_heights {
             let exp_blockhash = blocks.get(height).expect("block must exist in bitcoind");
             assert_eq!(
-                chain.blocks().get(height),
-                Some(exp_blockhash),
+                chain.get(*height).map(|cp| cp.hash()),
+                Some(*exp_blockhash),
                 "[{}:{}] block {}:{} must exist in final chain",
                 i,
                 t.name,