feat: restructure plugin and options in listconfigs

This will change the command `listconfigs` output in two ways:

 - remove duplicated "plugin" JSON output by replacing it with
 - a "plugins" substructure which maps plugin cmd/name to their options

Changelog-Changed: JSON-RPC: `listconfigs` now structures plugins and include their options

lightningd/plugin.c

@@ -1086,9 +1086,24 @@ void json_add_opt_plugins(struct json_stream *response,
  const struct plugins *plugins)
 {
  struct plugin *p;
+ struct plugin_opt *opt;
+ const char *name;
+
+ // we output 'plugins' and their config as substructure:
+ // "plugins" : { "name" -> { "param" : "value" }, ... }
+ json_object_start(response, "plugins");
  list_for_each(&plugins->plugins, p, list) {
- json_add_string(response, "plugin", p->cmd);
+ json_object_start(response, p->cmd);
+ list_for_each(&p->plugin_opts, opt, list) {
+ /* Trim the `--` that we added before */
+ name = opt->name + 2;
+ if (opt->value->as_str) {
+ json_add_string(response, name, opt->value->as_str);
+ }
+ }
+ json_object_end(response);
  }
+ json_object_end(response);
 }
 
 /**