Timed request scheduler

[Fadarrizz]

This PR adds a new scheduler that schedules requests based on their readyAt times. Scheduling requests this way makes retying with an exponential backoff a future possibility.

TimedRequestScheduler uses a priority queue to sort requests based on their readyAt times. If no readyAt time is available, the current time is used.

The order of requests with identical readyAt times cannot be guaranteed, due to the implemenation of the priority queue. Below is from the Php docs:

The order of elements with identical priority is undefined. It may differ from the order in which they have been inserted.
https://www.php.net/manual/en/class.splpriorityqueue.php

Making this the default scheduler could potentially break backwards compatibility because of the undefined order.

[ksassnowski]

Thanks for the contribution, but I think I'll hold off of adding this to the package itself for now. This is something that can reasonably done in user space via an external package. Instead of extending the Request class itself, you could use the request's meta information to store the readyAt information (see https://roach-php.dev/docs/processing-responses/#storing-meta-data)

[Fadarrizz]

Alright, seems fair. Thanks for the tips and checking it out!