Refactor EmailTemplateDto and update command handler

The `EmailTemplateDto` class has been changed from a record with positional parameters to a class with explicit properties, including XML documentation and required modifiers for `Name`, `Body`, and `Subject`.

In the `ResetEnvelopeTemplateCommandHandler`, added necessary using directives, modified the constructor to accept an `IRepository<EmailTemplate>`, and updated the `Handle` method to read and update email templates based on the request's ID or type. The static `Default` collection has been renamed to `Defaults` and now uses `EmailTemplateDto`.

EnvelopeGenerator.Application/DTOs/EmailTemplateDto.cs

@@ -3,10 +3,30 @@ using Microsoft.AspNetCore.Mvc;
 
 namespace EnvelopeGenerator.Application.DTOs
 {
+    /// <summary>
+    /// 
+    /// </summary>
     [ApiExplorerSettings(IgnoreApi = true)]
-    public record EmailTemplateDto(
-        int Id,
-        string Name,
-        string Body,
-        string Subject) : IUnique<int>;
+    public record EmailTemplateDto : IUnique<int>
+    {
+        /// <summary>
+        /// 
+        /// </summary>
+        public int Id{ get; init; }
+
+        /// <summary>
+        /// 
+        /// </summary>
+        public required string Name { get; init; }
+
+        /// <summary>
+        /// 
+        /// </summary>
+        public required string Body { get; init; }
+
+        /// <summary>
+        /// 
+        /// </summary>
+        public required string Subject { get; init; }
+    };
 }