[5.8] Add schedule event helpers to deal with successful/failed commands

[kbond]

Now that the exit code is available for scheduled command events (#28140), it might be worth having the following helpers available.

protected function schedule(Schedule $schedule)
{
    $schedule->command('my:command')
        ->onSuccess(function () {
            // do something when scheduled command ran successfully
        })
        ->onFailure(function () {
            // do something when scheduled command failed
        })
        ->pingOnSuccess('https://example.com')
        ->pingOnFailure('https://example.com')
        ->emailOnFailure('johndoe@example.com')
    ;
}

[taylorotwell]

Can you send a PR to the documentation?