Add <datadir>/settings.json persistent settings storage.

Persistent settings are used in followup PRs #15936 to unify gui settings
between bitcoin-qt and bitcoind, and #15937 to add a load_on_startup flag to
the loadwallet RPC and maintain a dynamic list of wallets that should be loaded
on startup that also can be shared between bitcoind and bitcoin-qt.

src/interfaces/node.cpp

@@ -66,6 +66,7 @@ public:
     bool softSetArg(const std::string& arg, const std::string& value) override { return gArgs.SoftSetArg(arg, value); }
     bool softSetBoolArg(const std::string& arg, bool value) override { return gArgs.SoftSetBoolArg(arg, value); }
     void selectParams(const std::string& network) override { SelectParams(network); }
+    bool initSettings(std::string& error) override { return gArgs.InitSettings(error); }
     uint64_t getAssumedBlockchainSize() override { return Params().AssumedBlockchainSize(); }
     uint64_t getAssumedChainStateSize() override { return Params().AssumedChainStateSize(); }
     std::string getNetwork() override { return Params().NetworkIDString(); }

src/interfaces/node.h

@@ -66,6 +66,11 @@ public:
     //! Choose network parameters.
     virtual void selectParams(const std::string& network) = 0;
 
+    //! Read and update <datadir>/settings.json file with saved settings. This
+    //! needs to be called after selectParams() because the settings file
+    //! location is network-specific.
+    virtual bool initSettings(std::string& error) = 0;
+
     //! Get the (assumed) blockchain size.
     virtual uint64_t getAssumedBlockchainSize() = 0;