feat: add first volunteer skill endpoint (add_skill) along with dtos

WebApp/DTOs/SkillSummaryDto.cs

@@ -0,0 +1,9 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace WebApp.DTOs;
+
+public record class SkillSummaryDto
+(
+    [Required] int SkillId,
+    [Required] string SkillName
+);