Add API for internal addresses

There are good reasons for applications to need to get internal
addresses too. For example creating a transactions that splits an output
into several smaller ones.

src/testutils/blockchain_tests.rs

@@ -815,7 +815,7 @@ macro_rules! bdk_blockchain_tests {
 
                 let mut builder = wallet.build_fee_bump(details.txid).unwrap();
                 builder.fee_rate(FeeRate::from_sat_per_vb(2.1));
-                let (mut new_psbt, new_details) = builder.finish().unwrap();
+                let (mut new_psbt, new_details) = builder.finish().expect("fee bump tx");
                 let finalized = wallet.sign(&mut new_psbt, Default::default()).unwrap();
                 assert!(finalized, "Cannot finalize transaction");
                 wallet.broadcast(&new_psbt.extract_tx()).unwrap();