Implemented resume_gateway_url. (#2423)

* Implemented resume_gateway_url.

* Made the requested changes.

* Implemented passing the gateway URL down from DiscordShardedClient.

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

src/Discord.Net.WebSocket/API/Gateway/ReadyEvent.cs

@@ -20,6 +20,8 @@ namespace Discord.API.Gateway
         public User User { get; set; }
         [JsonProperty("session_id")]
         public string SessionId { get; set; }
+        [JsonProperty("resume_gateway_url")]
+        public string ResumeGatewayUrl { get; set; }
         [JsonProperty("read_state")]
         public ReadState[] ReadStates { get; set; }
         [JsonProperty("guilds")]