Add new Backend structure with proper .NET 8 projects

src/Backend/DiunaBI.Core/Services/GoogleSheetsHelper.cs

@@ -0,0 +1,36 @@
+using Google.Apis.Auth.OAuth2;
+using Google.Apis.Services;
+using Google.Apis.Sheets.v4;
+using System.IO;
+
+namespace DiunaBI.Core.Services;
+
+public class GoogleSheetsHelper
+{
+    public SheetsService? Service { get; private set; }
+    private const string ApplicationName = "Diuna";
+    private static readonly string[] Scopes = [SheetsService.Scope.Spreadsheets];
+    public GoogleSheetsHelper()
+    {
+        InitializeService();
+    }
+    private void InitializeService()
+    {
+        var credential = GetCredentialsFromFile();
+        Service = new SheetsService(new BaseClientService.Initializer
+        {
+            HttpClientInitializer = credential,
+            ApplicationName = ApplicationName
+        });
+    }
+    private static GoogleCredential GetCredentialsFromFile()
+    {
+        var fileName = "client_secrets.json";
+#if DEBUG
+        fileName = "client_secrets.Development.json";
+#endif
+        using var stream = new FileStream(fileName, FileMode.Open, FileAccess.Read);
+        var credential = GoogleCredential.FromStream(stream).CreateScoped(Scopes);
+        return credential;
+    }
+}