[Feature] Support for the new username system (#2696)

* update a lot of stuff

* update CDN for new avatar calculation

* whoops, forgot to commit

* handle `null` values

* Remove duplicate line

Co-authored-by: Dmitry <dimson-n@users.noreply.github.com>

* updates

* Apply suggestions from code review

Co-authored-by: Quin Lynch <49576606+quinchs@users.noreply.github.com>

* Apply suggestion

Co-authored-by: Quin Lynch <49576606+quinchs@users.noreply.github.com>

* update `SocketThreadUSer`

---------

Co-authored-by: Dmitry <dimson-n@users.noreply.github.com>
Co-authored-by: Quin Lynch <49576606+quinchs@users.noreply.github.com>

src/Discord.Net.Rest/API/Common/User.cs

@@ -19,6 +19,9 @@ namespace Discord.API
         [JsonProperty("accent_color")]
         public Optional<uint?> AccentColor { get; set; }
 
+        [JsonProperty("global_name")]
+        public Optional<string> GlobalName { get; set; }
+
         //CurrentUser
         [JsonProperty("verified")]
         public Optional<bool> Verified { get; set; }