feat: ETLng loader basics (#1808)

For #1597

src/etlng/LoaderInterface.hpp

@@ -0,0 +1,52 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2024, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#pragma once
+
+#include "etlng/Models.hpp"
+
+#include <xrpl/protocol/LedgerHeader.h>
+
+#include <optional>
+
+namespace etlng {
+
+/**
+ * @brief An interface for a ETL Loader
+ */
+struct LoaderInterface {
+    virtual ~LoaderInterface() = default;
+
+    /**
+     * @brief Load ledger data
+     * @param data The data to load
+     */
+    virtual void
+    load(model::LedgerData const& data) = 0;
+
+    /**
+     * @brief Load the initial ledger
+     * @param data The data to load
+     * @return Optional ledger header
+     */
+    virtual std::optional<ripple::LedgerHeader>
+    loadInitialLedger(model::LedgerData const& data) = 0;
+};
+
+}  // namespace etlng