feat: add API controllers for template and channel management

- ApiResponse<T>, ApiError in Api/Infrastructure
- ApiControllerBase with IMediator, FromServiceResult, MapServiceError
- EmailTemplatesController: GET list, GET one, POST, PUT, DELETE
- EmailChannelsController: GET list, GET one, POST, PUT, DELETE

Ref: IT-628

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

HrynCo.NotificationService.Api/Controllers/EmailTemplates/CreateEmailTemplateRequest.cs

@@ -0,0 +1,13 @@
+using HrynCo.NotificationService.DAL.Abstract.Templates;
+
+namespace HrynCo.NotificationService.Api.Controllers.EmailTemplates;
+
+public sealed record CreateEmailTemplateRequest(
+    string ServiceName,
+    string Key,
+    string LanguageCode,
+    string Subject,
+    string HtmlBody,
+    string TextBody,
+    IReadOnlyList<EmailTemplateVariable> Variables
+);