Working on the UserRepository functionality

+ Tests
+ QueryableExtensions. It is an add-on that allows you to implement exceptions if the returned List would be empty.

Govor.Data/Repositories/Exceptions/NotFoundByKeyException.cs

@@ -0,0 +1,14 @@
+namespace Govor.Data.Repositories.Exceptions;
+
+public class NotFoundByKeyException<T> : NotFoundException
+{
+    public NotFoundByKeyException(T key) 
+        : base($"Not found object by {key}"){}
+
+    public NotFoundByKeyException(T id, string message) 
+        : base($"Not found object by {id}. Message: " + message) {}
+    
+    public NotFoundByKeyException(string message, Exception innerException) : base(message, innerException)
+    {
+    }
+}