Modules: introduced js_periodic directive.

Modules: introduced js_periodic directive.

The directive specifies a JS handler to run at regular intervals. The JS
handler will be executed in each worker process. The handler receives no
arguments. It has access to ngx and other global objects.

example.conf:
    location @periodics {
        # Specifies a JS handler to be run at 1 minute intervals
        js_periodic main.handler interval=60s jitter=5s;

        resolver 10.0.0.1;
        js_fetch_trusted_certificate /path/to/ISRG_Root_X1.pem;
    }

example.js:
    async function handler() {
        if (ngx.worker_id != 0) {
            /* using ngx.worker_id to run handler only in one worker. */
            return;
        }

        let reply = async ngx.fetch('https://nginx.org/en/docs/njs/');
        let body = async reply.text();

        ngx.log(ngx.INFO, body);
    }

This closes #660 issue on Github.