Moved files from backoffice into Common + Introduced AspNetCoreComponent to invoke the Umbraco Application Init event

src/Umbraco.Web.Common/AspNetCore/AspNetCoreSessionIdResolver.cs

@@ -0,0 +1,30 @@
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Http.Features;
+using Umbraco.Net;
+
+namespace Umbraco.Web.Common.AspNetCore
+{
+    internal class AspNetCoreSessionIdResolver : ISessionIdResolver
+    {
+        private readonly IHttpContextAccessor _httpContextAccessor;
+
+        public AspNetCoreSessionIdResolver(IHttpContextAccessor httpContextAccessor)
+        {
+            _httpContextAccessor = httpContextAccessor;
+        }
+
+
+        public string SessionId
+        {
+            get
+            {
+                var httpContext = _httpContextAccessor?.HttpContext;
+                // If session isn't enabled this will throw an exception so we check
+                var sessionFeature = httpContext?.Features.Get<ISessionFeature>();
+                return sessionFeature != null
+                    ? httpContext?.Session?.Id
+                    : "0";
+            }
+        }
+    }
+}