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/rpc/client.cpp

@@ -173,6 +173,9 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "createwallet", 2, "blank"},
     { "createwallet", 4, "avoid_reuse"},
     { "createwallet", 5, "descriptors"},
+    { "createwallet", 6, "load_on_startup"},
+    { "loadwallet", 1, "load_on_startup"},
+    { "unloadwallet", 1, "load_on_startup"},
     { "getnodeaddresses", 0, "count"},
     { "addpeeraddress", 1, "port"},
     { "stop", 0, "wait" },