Refactor recent messages API (#4763)

This exposes internal functions for testing by splitting the implementation & internal API into separate files

src/providers/recentmessages/Api.hpp

@@ -0,0 +1,35 @@
+#pragma once
+
+#include <QString>
+
+#include <functional>
+#include <memory>
+#include <vector>
+
+namespace chatterino {
+
+class Channel;
+using ChannelPtr = std::shared_ptr<Channel>;
+
+struct Message;
+using MessagePtr = std::shared_ptr<const Message>;
+
+}  // namespace chatterino
+
+namespace chatterino::recentmessages {
+
+using ResultCallback = std::function<void(const std::vector<MessagePtr> &)>;
+using ErrorCallback = std::function<void()>;
+
+/**
+ * @brief Loads recent messages for a channel using the Recent Messages API
+ *
+ * @param channelName Name of Twitch channel
+ * @param channelPtr Weak pointer to Channel to use to build messages
+ * @param onLoaded Callback taking the built messages as a const std::vector<MessagePtr> &
+ * @param onError Callback called when the network request fails
+ */
+void load(const QString &channelName, std::weak_ptr<Channel> channelPtr,
+          ResultCallback onLoaded, ErrorCallback onError);
+
+}  // namespace chatterino::recentmessages