lnrpc: add macaroon_root_key to InitWalletRequest

This adds a field for specifying the macaroon root key to use during the
wallet initialization process instead of having lnd generate a random
macaroon root key itself.

lnrpc/walletunlocker.proto

@@ -185,6 +185,13 @@ message InitWalletRequest {
     corresponding private keys and can serve signing RPC requests.
     */
     WatchOnly watch_only = 9;
+
+    /*
+    macaroon_root_key is an optional 32 byte macaroon root key that can be
+    provided when initializing the wallet rather than letting lnd generate one
+    on its own.
+    */
+    bytes macaroon_root_key = 10;
 }
 message InitWalletResponse {
     /*