implemented Discord OAuth2

Lieb/DiscordOAuth2/DiscordHandler.cs

@@ -0,0 +1,36 @@
+using Microsoft.AspNetCore.Authentication;
+using Microsoft.AspNetCore.Authentication.OAuth;
+using Microsoft.Extensions.Options;
+using System.Net.Http.Headers;
+using System.Security.Claims;
+using System.Text.Encodings.Web;
+using System.Text.Json;
+
+namespace Discord.OAuth2
+{
+    internal class DiscordHandler : OAuthHandler<DiscordOptions>
+    {
+        public DiscordHandler(IOptionsMonitor<DiscordOptions> options, ILoggerFactory logger, UrlEncoder encoder, ISystemClock clock)
+            : base(options, logger, encoder, clock)
+        {
+        }
+
+        protected override async Task<AuthenticationTicket> CreateTicketAsync(ClaimsIdentity identity, AuthenticationProperties properties, OAuthTokenResponse tokens)
+        {
+            var request = new HttpRequestMessage(HttpMethod.Get, Options.UserInformationEndpoint);
+            request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", tokens.AccessToken);
+            request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
+
+            var response = await Backchannel.SendAsync(request, Context.RequestAborted);
+            if (!response.IsSuccessStatusCode)
+                throw new HttpRequestException($"Failed to retrieve Discord user information ({response.StatusCode}).");
+
+            var payload = JsonDocument.Parse(await response.Content.ReadAsStringAsync());
+            var context = new OAuthCreatingTicketContext(new ClaimsPrincipal(identity), properties, Context, Scheme, Options, Backchannel, tokens, payload.RootElement);
+            context.RunClaimActions();
+
+            await Events.CreatingTicket(context);
+            return new AuthenticationTicket(context.Principal, context.Properties, Scheme.Name);
+        }
+    }
+}