fees: add ForecasterMan class

- Its a module for managing and utilising multiple
  fee rate forecasters to provide fee rate forecast.

- The ForecasterManager class allows for the registration of
  multiple fee rate forecasters.

Co-authored-by: willcl-ark <will@256k1.dev>

src/CMakeLists.txt

@@ -259,6 +259,7 @@ add_library(bitcoin_node STATIC EXCLUDE_FROM_ALL
   policy/ephemeral_policy.cpp
   policy/fees.cpp
   policy/fees_args.cpp
+  policy/fees/forecaster_man.cpp
   policy/packages.cpp
   policy/rbf.cpp
   policy/settings.cpp

src/policy/fees/forecaster_man.cpp

@@ -0,0 +1,14 @@
+// Copyright (c) 2025 The Bitcoin Core developers
+// Distributed under the MIT software license. See the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <policy/fees/forecaster.h>
+#include <policy/fees/forecaster_man.h>
+#include <policy/fees/forecaster_util.h>
+
+#include <utility>
+
+void FeeRateForecasterManager::RegisterForecaster(std::shared_ptr<Forecaster> forecaster)
+{
+    forecasters.emplace(forecaster->GetForecastType(), forecaster);
+}

src/policy/fees/forecaster_man.h

@@ -0,0 +1,37 @@
+// Copyright (c) 2025 The Bitcoin Core developers
+// Distributed under the MIT software license. See the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_POLICY_FEES_FORECASTER_MAN_H
+#define BITCOIN_POLICY_FEES_FORECASTER_MAN_H
+
+#include <memory>
+#include <unordered_map>
+
+class Forecaster;
+class ForecastResult;
+
+enum class ForecastType;
+
+/** \class FeeRateForecasterManager
+ * Module for managing and utilising multiple fee rate forecasters to provide a forecast for a target.
+ *
+ * The FeeRateForecasterManager class allows for the registration of multiple fee rate
+ * forecasters.
+ */
+class FeeRateForecasterManager
+{
+private:
+    //! Map of all registered forecasters to their shared pointers.
+    std::unordered_map<ForecastType, std::shared_ptr<Forecaster>> forecasters;
+
+public:
+    /**
+     * Register a forecaster to provide fee rate estimates.
+     *
+     * @param[in] forecaster shared pointer to a Forecaster instance.
+     */
+    void RegisterForecaster(std::shared_ptr<Forecaster> forecaster);
+};
+
+#endif // BITCOIN_POLICY_FEES_FORECASTER_MAN_H