Add OAuth2 support

Oqtane.Server/Pages/External.cshtml.cs

@@ -0,0 +1,29 @@
+using System.Net;
+using Microsoft.AspNetCore.Authentication;
+using Microsoft.AspNetCore.Authentication.OpenIdConnect;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using Oqtane.Extensions;
+
+namespace Oqtane.Pages
+{
+    public class ExternalModel : PageModel
+    {
+        public IActionResult OnGetAsync(string returnurl)
+        {
+            returnurl = (returnurl == null) ? "/" : returnurl;
+            returnurl = (!returnurl.StartsWith("/")) ? "/" + returnurl : returnurl;
+
+            var providertype = HttpContext.GetAlias().SiteSettings.GetValue("ExternalLogin:ProviderType", "");
+            if (providertype != "")
+            {
+                    return new ChallengeResult(providertype, new AuthenticationProperties { RedirectUri = returnurl });
+            }
+            else
+            {
+                HttpContext.Response.StatusCode = (int)HttpStatusCode.Forbidden;
+                return new EmptyResult();
+            }
+        }
+    }
+}