C#: Add source generator for signals as events

Changed the signal declaration signal to:

```
// The following generates a MySignal event
[Signal] public delegate void MySignalEventHandler(int param);
```

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Sample/EventSignals.cs

@@ -0,0 +1,7 @@
+namespace Godot.SourceGenerators.Sample;
+
+public partial class EventSignals : Godot.Object
+{
+    [Signal]
+    public delegate void MySignalEventHandler(string str, int num);
+}