Added more aspnet core implementation of interfaces

src/Umbraco.Web.BackOffice/AspNetCore/AspNetCoreUserAgentProvider.cs

@@ -0,0 +1,20 @@
+using Microsoft.AspNetCore.Http;
+using Umbraco.Net;
+
+namespace Umbraco.Web.BackOffice.AspNetCore
+{
+    public class AspNetCoreUserAgentProvider : IUserAgentProvider
+    {
+        private readonly IHttpContextAccessor _httpContextAccessor;
+
+        public AspNetCoreUserAgentProvider(IHttpContextAccessor httpContextAccessor)
+        {
+            _httpContextAccessor = httpContextAccessor;
+        }
+
+        public string GetUserAgent()
+        {
+            return _httpContextAccessor.HttpContext.Request.Headers["User-Agent"].ToString();
+        }
+    }
+}