feat(helix): add startCommercial endpoint

Sources/Twitch/API/Endpoints/Ads/StartCommercial.swift

@@ -0,0 +1,30 @@
+import Foundation
+
+#if canImport(FoundationNetworking)
+  import FoundationNetworking
+#endif
+
+extension Helix {
+  public func startCommercial(broadcasterId: String, length: Int) async throws
+    -> Commercial?
+  {
+    var queryItems = [URLQueryItem]()
+    queryItems.append(
+      URLQueryItem(name: "broadcaster_id", value: broadcasterId))
+    queryItems.append(URLQueryItem(name: "length", value: String(length)))
+
+    return try await self.request(.get("channels"), with: queryItems).first
+  }
+}
+
+public struct Commercial: Codable {
+  let length: Int
+  let message: String
+  let retryAfter: Int
+
+  enum CodingKeys: String, CodingKey {
+    case length
+    case message
+    case retryAfter = "retry_after"
+  }
+}