move-only: move warnings from common to node

Since rpc/util.cpp is in common, also move GetNodeWarnings() to
node::GetWarningsForRPC()

src/node/warnings.h

@@ -0,0 +1,31 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-2021 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_NODE_WARNINGS_H
+#define BITCOIN_NODE_WARNINGS_H
+
+#include <optional>
+#include <string>
+#include <vector>
+
+class UniValue;
+struct bilingual_str;
+
+namespace node {
+void SetMiscWarning(const bilingual_str& warning);
+void SetfLargeWorkInvalidChainFound(bool flag);
+/** Pass std::nullopt to disable the warning */
+void SetMedianTimeOffsetWarning(std::optional<bilingual_str> warning);
+/** Return potential problems detected by the node. */
+std::vector<bilingual_str> GetWarnings();
+/**
+ * RPC helper function that wraps GetWarnings. Returns a UniValue::VSTR
+ * with the latest warning if use_deprecated is set to true, or a
+ * UniValue::VARR with all warnings otherwise.
+ */
+UniValue GetWarningsForRpc(bool use_deprecated);
+} // namespace node
+
+#endif // BITCOIN_NODE_WARNINGS_H