[8.x] Support delaying notifications per channel

[iamgergo]

Recently I faced the situation, I had to delay the same notification differently on database/email/sms channels.

Currently, there is no easy way to do that, however, we could solve this quite simply. This is a working, but rather conceptional PR, if you find this approach useful, we might polish it further, refactor a bit, update the docblocks, maybe extract the channel delay to its own method, adding tests and so on.

---

User::first()->notify(
    (new NewOrder(Order::first()))->delay([
        'mail' => now()->addDay(),
        'sms' => 300,
    ])
);

• This would not be a breaking change – as far as I know
• If a channel is not present in the delayed values it won't be delayed
• If the value is not an array, the given delay will be used by all channels (like now)

[taylorotwell]

Thank you. Can you send a PR to the documentation?

[iamgergo]

Yes, I'll send it soon. Thanks.