Support post logout redirect state parameter (#193)

Expose post logout redirect state parameter. Already supported by IdentityModel.

Author: woody3k

src/AuthorizeClient.cs

@@ -119,7 +119,8 @@ internal string CreateEndSessionUrl(string endpoint, LogoutRequest request)
 
             return new RequestUrl(endpoint).CreateEndSessionUrl(
                 idTokenHint: request.IdTokenHint,
-                postLogoutRedirectUri: _options.PostLogoutRedirectUri);
+                postLogoutRedirectUri: _options.PostLogoutRedirectUri,
+								state: request.State);
         }
 
         internal Dictionary<string, string> CreateAuthorizeParameters(string state, string nonce, string codeChallenge, IDictionary<string, string> extraParameters)

src/Requests/LogoutRequest.cs

@@ -34,5 +34,13 @@ public class LogoutRequest
         /// The identifier token hint.
         /// </value>
         public string IdTokenHint { get; set; }
+
+        /// <summary>
+        /// Gets or sets the state.
+        /// </summary>
+        /// <value>
+        /// The state value passed back to client as query string on post_logout_redirect_uri
+        /// </value>
+        public string State { get; set; }
     }
 }