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

WebApp/Mapping/VolunteerSkillMapping.cs

@@ -0,0 +1,17 @@
+using WebApp.DTOs;
+using WebApp.Entities;
+
+namespace WebApp.Mapping
+{
+    public static class VolunteerSkillMapping
+    {
+        public static VolunteerSkill ToVolunteerSkillEntity(this SingleSkillDto SSDto, int uid)
+        {
+            return new VolunteerSkill()
+            {
+                UserId = uid,
+                SkillId = SSDto.Skill,
+            };
+        }
+    }
+}