Adds global.json file for .NET SDK configuration

Adds a global.json file to specify the .NET SDK version and rollForward policy.
This ensures consistent builds and development environments by pinning the SDK version.

Author: Foxlider

global.json

@@ -0,0 +1,6 @@
+{
+  "sdk": {
+    "rollForward": "latestMinor",
+    "version": "9.0.0"
+  }
+}