Refactor email template query handling with MediatR

- Updated `ReadEmailTemplateQuery` to implement `IRequest<ReadEmailTemplateResponse?>`.
- Changed `ReadEmailTemplateResponse` from a record to a class with updated properties.
- Enhanced `EmailTemplateController` to inject `IEmailTemplateRepository` and `IMediator`, and made the `Get` method asynchronous.
- Introduced `ReadEmailTemplateMappingProfile` for AutoMapper mappings.
- Added `ReadEmailTemplateQueryHandler` to manage query logic and response mapping.

These changes improve the structure and maintainability of the email template querying process.

EnvelopeGenerator.Application/EmailTemplates/Queries/Read/ReadEmailTemplateMappingProfile.cs

@@ -0,0 +1,23 @@
+using AutoMapper;
+using EnvelopeGenerator.Domain.Entities;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace EnvelopeGenerator.Application.EmailTemplates.Queries.Read;
+
+/// <summary>
+/// 
+/// </summary>
+public class ReadEmailTemplateMappingProfile : Profile
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    public ReadEmailTemplateMappingProfile()
+    {
+        CreateMap<EmailTemplate, ReadEmailTemplateResponse>();
+    }
+}