add csharp snippets

snippets/csharp/SendSpontaneousPayment.cs

@@ -0,0 +1,21 @@
+using Breez.Sdk;
+
+public class SendSpontaneousPaymentSnippets
+{
+    public void SendSpontaneousPayment(BlockingBreezServices sdk)
+    {
+        // ANCHOR: send-spontaneous-payment
+        var nodeId = "...";
+        ulong amountMsat = 3_000_000;
+        try
+        {
+            var response = sdk.SendSpontaneousPayment(
+                new SendSpontaneousPaymentRequest(nodeId, amountMsat));
+        }
+        catch (Exception)
+        {
+            // Handle error
+        }
+        // ANCHOR_END: send-spontaneous-payment
+    }
+}