lnwallet+lnrpc: add address index to ListAddresses

For the itest in the next commit we'll need to be able to fetch the
input information for an address over RPC. The only piece missing is the
address' index, which we add in this commit. Everything else should be
derivable from the ListAddresses and ListAccounts calls.

lnwallet/interface.go

@@ -96,6 +96,12 @@ type AddressProperty struct {
 
 	// Balance returns the total balance of an address.
 	Balance btcutil.Amount
+
+	// DerivationPath is the derivation path of the address.
+	DerivationPath string
+
+	// PublicKey is the public key of the address.
+	PublicKey *btcec.PublicKey
 }
 
 // AccountIdentifier contains information to uniquely identify an account.