[bdk_chain_redesign] Introduce ChainOracle and TxIndex traits

The chain oracle keeps track of the best chain, while the transaction
index indexes transaction data in relation to script pubkeys.

This commit also includes initial work on `IndexedTxGraph`.

crates/chain/src/keychain/txout_index.rs

@@ -1,7 +1,7 @@
 use crate::{
     collections::*,
     miniscript::{Descriptor, DescriptorPublicKey},
-    ForEachTxOut, SpkTxOutIndex,
+    ForEachTxOut, SpkTxOutIndex, TxIndex,
 };
 use alloc::{borrow::Cow, vec::Vec};
 use bitcoin::{secp256k1::Secp256k1, OutPoint, Script, TxOut};
@@ -88,6 +88,22 @@ impl<K> Deref for KeychainTxOutIndex<K> {
     }
 }
 
+impl<K: Clone + Ord + Debug> TxIndex for KeychainTxOutIndex<K> {
+    type Additions = DerivationAdditions<K>;
+
+    fn index_txout(&mut self, outpoint: OutPoint, txout: &TxOut) -> Self::Additions {
+        self.scan_txout(outpoint, txout)
+    }
+
+    fn index_tx(&mut self, tx: &bitcoin::Transaction) -> Self::Additions {
+        self.scan(tx)
+    }
+
+    fn is_tx_relevant(&self, tx: &bitcoin::Transaction) -> bool {
+        self.is_relevant(tx)
+    }
+}
+
 impl<K: Clone + Ord + Debug> KeychainTxOutIndex<K> {
     /// Scans an object for relevant outpoints, which are stored and indexed internally.
     ///