Add loadwallet and createwallet RPC load_on_startup options

This maintains a persistent list of wallets stored in settings that will
automatically be loaded on startup. Being able to load a wallet automatically
on startup will be more useful in the GUI when the option to create wallets is
added in #15006, but it's reasonable to expose this feature by RPC as well.

src/interfaces/chain.h

@@ -7,6 +7,7 @@
 
 #include <optional.h>               // For Optional and nullopt
 #include <primitives/transaction.h> // For CTransactionRef
+#include <util/settings.h>          // For util::SettingsValue
 
 #include <functional>
 #include <memory>
@@ -269,6 +270,12 @@ public:
     //! Current RPC serialization flags.
     virtual int rpcSerializationFlags() = 0;
 
+    //! Return <datadir>/settings.json setting value.
+    virtual util::SettingsValue getRwSetting(const std::string& name) = 0;
+
+    //! Write a setting to <datadir>/settings.json.
+    virtual bool updateRwSetting(const std::string& name, const util::SettingsValue& value) = 0;
+
     //! Synchronously send transactionAddedToMempool notifications about all
     //! current mempool transactions to the specified handler and return after
     //! the last one is sent. These notifications aren't coordinated with async