Merge #15935: Add <datadir>/settings.json persistent settings storage

9c69cfe4c5 Add <datadir>/settings.json persistent settings storage. (Russell Yanofsky)
eb682c5700 util: Add ReadSettings and WriteSettings functions (Russell Yanofsky)

Pull request description:

  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.

ACKs for top commit:
  MarcoFalke:
    Approach re-ACK 9c69cfe4c5 🌾
  jnewbery:
    utACK 9c69cfe4c5

Tree-SHA512: 39fcc6051717117c9141e934de1d0d3f739484be4685cdf97d54de967c8c816502b4fd0de12114433beaa5c5b7060c810fd8ae4e2b3ce7c371eb729ac01ba2e1

doc/files.md

@@ -50,7 +50,7 @@ Subdirectory       | File(s)               | Description
 `indexes/blockfilter/basic/`    | `fltrNNNNN.dat`<sup>[\[2\]](#note2)</sup> | Blockfilter index filters for the basic filtertype; *optional*, used if `-blockfilterindex=basic`
 `wallets/`         |                       | [Contains wallets](#multi-wallet-environment); can be specified by `-walletdir` option; if `wallets/` subdirectory does not exist, a wallet resides in the data directory
 `./`               | `banlist.dat`         | Stores the IPs/subnets of banned nodes
-`./`               | `bitcoin.conf`        | Contains [configuration settings](bitcoin-conf.md) for `bitcoind` or `bitcoin-qt`; can be specified by `-conf` option
+`./`               | `bitcoin.conf`        | User-defined [configuration settings](bitcoin-conf.md) for `bitcoind` or `bitcoin-qt`. File is not written to by the software and must be created manually. Path can be specified by `-conf` option
 `./`               | `bitcoind.pid`        | Stores the process ID (PID) of `bitcoind` or `bitcoin-qt` while running; created at start and deleted on shutdown; can be specified by `-pid` option
 `./`               | `debug.log`           | Contains debug information and general logging generated by `bitcoind` or `bitcoin-qt`; can be specified by `-debuglogfile` option
 `./`               | `fee_estimates.dat`   | Stores statistics used to estimate minimum transaction fees and priorities required for confirmation
@@ -58,6 +58,7 @@ Subdirectory       | File(s)               | Description
 `./`               | `mempool.dat`         | Dump of the mempool's transactions
 `./`               | `onion_private_key`   | Cached Tor hidden service private key for `-listenonion` option
 `./`               | `peers.dat`           | Peer IP address database (custom format)
+`./`               | `settings.json`       | Read-write settings set through GUI or RPC interfaces, augmenting manual settings from [bitcoin.conf](bitcoin-conf.md). File is created automatically if read-write settings storage is not disabled with `-nosettings` option. Path can be specified with `-settings` option
 `./`               | `.cookie`             | Session RPC authentication cookie; if used, created at start and deleted on shutdown; can be specified by `-rpccookiefile` option
 `./`               | `.lock`               | Data directory lock file