fix: inherit EF entities from Entity base class

- TemplateEntity, ProviderEntity, ProviderUsageEntity now inherit Entity (from DAL.Abstract)
- Removes duplicated Id, Created, Updated properties from each entity

Ref: IT-628

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

HrynCo.NotificationService.DAL.EF/Entities/TemplateEntity.cs

@@ -1,8 +1,9 @@
+using HrynCo.NotificationService.DAL.Abstract.Entities;
+
 namespace HrynCo.NotificationService.DAL.EF.Entities;
 
-internal class TemplateEntity
+internal class TemplateEntity : Entity
 {
-    public Guid Id { get; set; }
     public required string ServiceName { get; set; }
     public required string Key { get; set; }
     public required string LanguageCode { get; set; }
@@ -10,8 +11,6 @@ internal class TemplateEntity
     public required string HtmlBody { get; set; }
     public required string TextBody { get; set; }
     public List<TemplateVariableData> Variables { get; set; } = [];
-    public DateTimeOffset Created { get; set; }
-    public DateTimeOffset? Updated { get; set; }
 }
 
 internal class TemplateVariableData