electrs: add option 'extraArgs'

Electrs allows defining settings multiple times via cmdline args, but
not via config files.
So 'extraArgs' is the only way to implement overridable settings,
'extraOptions' wouldn't work.

modules/electrs.nix

@@ -49,6 +49,11 @@ in {
       default = 50002;
       description = "Port on which to listen for tor client connections.";
     };
+    extraArgs = mkOption {
+      type = types.separatedString " ";
+      default = "";
+      description = "Extra command line arguments passed to electrs.";
+    };
     TLSProxy = {
       enable = mkEnableOption "Nginx TLS proxy";
       port = mkOption {
@@ -81,7 +86,7 @@ in {
           ${pkgs.nix-bitcoin.electrs}/bin/electrs -vvv \
           ${optionalString (!cfg.high-memory) "--jsonrpc-import --index-batch-size=10"} \
           --db-dir '${cfg.dataDir}' --daemon-dir '${config.services.bitcoind.dataDir}' \
-          --electrum-rpc-addr=${toString cfg.address}:${toString cfg.port}
+          --electrum-rpc-addr=${toString cfg.address}:${toString cfg.port} ${cfg.extraArgs}
         '';
         User = cfg.user;
         Group = cfg.group;