feature: add Add Guild Member endpoint (#1183)

* Add AddGuildMember Oauth endpoint support

* Concat RoleIds if already exists.

* Use local ids variable.

src/Discord.Net.Rest/API/Rest/AddGuildMemberParams.cs

@@ -0,0 +1,19 @@
+using Newtonsoft.Json;
+
+namespace Discord.API.Rest
+{
+    [JsonObject(MemberSerialization = MemberSerialization.OptIn)]
+    internal class AddGuildMemberParams
+    {
+        [JsonProperty("access_token")]
+        public string AccessToken { get; set; }
+        [JsonProperty("nick")]
+        public Optional<string> Nickname { get; set; }
+        [JsonProperty("roles")]
+        public Optional<ulong[]> RoleIds { get; set; }
+        [JsonProperty("mute")]
+        public Optional<bool> IsMuted { get; set; }
+        [JsonProperty("deaf")]
+        public Optional<bool> IsDeafened { get; set; }
+    }
+}