multi: forward address and import calls over RPC

In order to support the full range of on-chain functionality, including
importing watch-only accounts in the watch-only instance, we need to
forward some calls like creating new addresses or importing accounts to
the remote signing instance.

lnrpc/walletrpc/walletkit.proto

@@ -286,6 +286,16 @@ message AddrRequest {
     default wallet account is used.
     */
     string account = 1;
+
+    /*
+    The type of address to derive.
+    */
+    AddressType type = 2;
+
+    /*
+    Whether a change address should be derived.
+    */
+    bool change = 3;
 }
 message AddrResponse {
     /*