[bdk_chain_redesign] Change insert_relevant_txs method

Instead of forcing all transactions inserted to use the same anchors, we
change the API to have unique anchors per transaction.

This allows for more flexibility in general. For example, use `Anchor`
implementations that contain the position in a block of a transaction.

crates/chain/tests/test_indexed_tx_graph.rs

@@ -61,7 +61,7 @@ fn insert_relevant_txs() {
     let txs = [tx_c, tx_b, tx_a];
 
     assert_eq!(
-        graph.insert_relevant_txs(&txs, None, None),
+        graph.insert_relevant_txs(txs.iter().map(|tx| (tx, None)), None),
         IndexedAdditions {
             graph_additions: Additions {
                 tx: txs.into(),