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.

test/functional/test_runner.py

@@ -243,6 +243,7 @@ BASE_SCRIPTS = [
     'p2p_permissions.py',
     'feature_blocksdir.py',
     'feature_config_args.py',
+    'feature_settings.py',
     'rpc_getdescriptorinfo.py',
     'rpc_getpeerinfo_banscore_deprecation.py',
     'rpc_help.py',