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.

doc/release-notes-15937.md

@@ -0,0 +1,12 @@
+Configuration
+-------------
+
+The `createwallet`, `loadwallet`, and `unloadwallet` RPCs now accept
+`load_on_startup` options that modify bitcoin's dynamic configuration in
+`\<datadir\>/settings.json`, and can add or remove a wallet from the list of
+wallets automatically loaded at startup. Unless these options are explicitly
+set to true or false, the load on startup wallet list is not modified, so this
+change is backwards compatible.
+
+In the future, the GUI will start updating the same startup wallet list as the
+RPCs to automatically reopen wallets previously opened in the GUI.