refactor(Extension): move to Application.Extensions and remove the project.

- update references

EnvelopeGenerator.Application/Extensions/StringExtension.cs

@@ -0,0 +1,14 @@
+using OtpNet;
+
+namespace EnvelopeGenerator.Application.Extensions
+{
+    public static class StringExtension
+    {
+        public static bool IsValidTotp(this string totp, string secret)
+        {
+            var secret_bytes = Base32Encoding.ToBytes(secret);
+            var secret_totp = new Totp(secret_bytes);
+            return secret_totp.VerifyTotp(totp, out _, VerificationWindow.RfcSpecifiedNetworkDelay);
+        }
+    }
+}