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.

lnrpc/walletrpc/walletkit.swagger.json

@@ -1317,6 +1317,15 @@
           "type": "string",
           "format": "int64",
           "description": "The balance of the address."
+        },
+        "derivation_path": {
+          "type": "string",
+          "description": "The full derivation path of the address. This will be empty for imported\naddresses."
+        },
+        "public_key": {
+          "type": "string",
+          "format": "byte",
+          "description": "The public key of the address. This will be empty for imported addresses."
         }
       }
     },