rpc: Move mempool RPCs to new file

Can be reviewed with:
--color-moved=dimmed-zebra --color-moved-ws=ignore-all-space

src/rpc/mempool.h

@@ -0,0 +1,17 @@
+// Copyright (c) 2017-2022 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_RPC_MEMPOOL_H
+#define BITCOIN_RPC_MEMPOOL_H
+
+class CTxMemPool;
+class UniValue;
+
+/** Mempool information to JSON */
+UniValue MempoolInfoToJSON(const CTxMemPool& pool);
+
+/** Mempool to JSON */
+UniValue MempoolToJSON(const CTxMemPool& pool, bool verbose = false, bool include_mempool_sequence = false);
+
+#endif // BITCOIN_RPC_MEMPOOL_H