feat: worker pinging

[AlliBalliBaba]

This PR is an idea I had to cover cases like #2467. It basically adds a way to send recurring messages to a worker via caddy config with relatively little code. This allows creating persistent schedulers, managing/cleaning up connections/caches and probably many other use cases.

# Caddyfile
worker /path/to/worker.php {
    ping overlap 1m "hello"
}

# /path/to/worker.php
while frankenphp_handle_request(function($message = '') {
    echo $message; # "hello"
})

I imagine this would also be possible as an extension, but it's more accessible here in core. Let me know what you think.