multi: add label to WalletController SendOutputs and dependent rpcs

Add a label parameter to the WalletController SendOutputs endpoint and
update rpcs that use it to allow optional provision of labels.

lnrpc/rpc.proto

@@ -1002,6 +1002,9 @@ message SendManyRequest {
     // A manual fee rate set in sat/byte that should be used when crafting the
     // transaction.
     int64 sat_per_byte = 5;
+
+    // An optional label for the transaction, limited to 500 characters.
+    string label = 6;
 }
 message SendManyResponse {
     // The id of the transaction
@@ -1029,6 +1032,9 @@ message SendCoinsRequest {
     address.
     */
     bool send_all = 6;
+
+    // An optional label for the transaction, limited to 500 characters.
+    string label = 7;
 }
 message SendCoinsResponse {
     // The transaction ID of the transaction

lnrpc/rpc.swagger.json

@@ -4230,6 +4230,10 @@
           "type": "boolean",
           "format": "boolean",
           "description": "If set, then the amount field will be ignored, and lnd will attempt to\nsend all the coins under control of the internal wallet to the specified\naddress."
+        },
+        "label": {
+          "type": "string",
+          "description": "An optional label for the transaction, limited to 500 characters."
         }
       }
     },

lnrpc/walletrpc/walletkit_server.go

@@ -311,6 +311,7 @@ func (w *WalletKit) SendOutputs(ctx context.Context,
 	// attempt to create this transaction.
 	tx, err := w.cfg.Wallet.SendOutputs(
 		outputsToCreate, chainfee.SatPerKWeight(req.SatPerKw),
+		labels.External,
 	)
 	if err != nil {
 		return nil, err