DiscordKit: can receive and decode hello event

Sources/DiscordKit/Models.swift

@@ -15,3 +15,36 @@ public struct SessionStartLimit: Codable {
     let reset_after: Int
     let max_concurrency: Int
 }
+
+public enum GatewayPayload: Decodable {
+    case hello(HelloPayload)
+}
+
+public struct GatewayMessage: Decodable {
+    let op: Int
+    let d: GatewayPayload?
+
+    enum CodingKeys: String, CodingKey {
+        case t
+        case s
+        case op
+        case d
+    }
+
+    public init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        op = try container.decode(Int.self, forKey: .op)
+        switch op {
+            case 10:
+                let hello = try container.decode(HelloPayload.self, forKey: .d)
+                d = .hello(hello)
+            default:
+                d = nil
+                break
+        }
+    }
+}
+
+public struct HelloPayload: Codable {
+    let heartbeat_interval: Int
+}