Add Nintendo Switch auth methods

Modio/Clients/AuthClient.cs

@@ -86,6 +86,15 @@ public async Task<AccessToken> External(XboxAuth options)
             return await RequestToken(route, options.ToContent());
         }
 
+        /// <summary>
+        /// Request an access token on behalf of a Nintendo Switch user.
+        /// </summary>
+        public async Task<AccessToken> External(SwitchAuth options)
+        {
+            var route = Routes.ExternalSwitch();
+            return await RequestToken(route, options.ToContent());
+        }
+
         /// <summary>
         /// Request an access token on behalf of a Discord user.
         /// </summary>

Modio/Http/Routes.cs

@@ -41,6 +41,11 @@ public static (HttpMethod, Uri) ExternalXbox()
             return (HttpMethod.Post, new Uri("external/xboxauth", UriKind.Relative));
         }
 
+        public static (HttpMethod, Uri) ExternalSwitch()
+        {
+            return (HttpMethod.Post, new Uri("external/switchauth", UriKind.Relative));
+        }
+
         public static (HttpMethod, Uri) ExternalDiscord()
         {
             return (HttpMethod.Post, new Uri("external/discordauth", UriKind.Relative));

Modio/Models/Request/Auth/SwitchAuth.cs

@@ -0,0 +1,51 @@
+using System.Net.Http;
+
+namespace Modio
+{
+    /// <summary>
+    /// See <see cref="AuthClient.External(SwitchAuth)"/>.
+    /// </summary>
+    ///
+    /// <seealso>https://docs.mod.io/#authenticate-via-switch</seealso>
+    public class SwitchAuth
+    {
+        /// <summary>
+        /// The NSA ID supplied by the Nintendo Switch SDK.
+        /// </summary>
+        public string IdToken { get; private set; }
+
+        /// <summary>
+        /// The users email address.
+        /// </summary>
+        public string? Email { get; set; }
+
+        /// <summary>
+        /// Unix timestamp of date in which the returned token will expire.
+        /// </summary>
+        public long? ExpiredAt { get; set; }
+
+        /// <summary>
+        /// Initializes a new instance of <see cref="SwitchAuth"/> with the required NSA ID.
+        /// </summary>
+        public SwitchAuth(string idToken)
+        {
+            IdToken = idToken;
+        }
+
+        internal HttpContent ToContent()
+        {
+            var parameters = new Parameters {
+                {"id_token", IdToken},
+            };
+            if (Email is string email)
+            {
+                parameters.Add("email", email);
+            }
+            if (ExpiredAt is long expiredAt)
+            {
+                parameters.Add("date_expires", expiredAt.ToString());
+            }
+            return parameters.ToContent();
+        }
+    }
+}