Allow for custom retry amount, delay, and timeout.

[AndreasBackx]

Lately our calls to GoCardless are taking an excessive amount of time and are even causing timeouts in our wsgi. Calls to BaseService._perform_request are all using hardcoded settings, e.g: RedirectFlowsService.create. It would be appreciated if we could configure these ourselves somehow and also configure a timeout. The library uses requests internally which allows you to pass a timeout, it would be neat if we could pass all this data ourselves and handle the timeout and other exceptions ourselves.

[timrogers]

@AndreasBackx Thanks for your feedback! As you may have seen, we've recently added new functionality to the library to support retrying failed requests automatically. It'd definitely make sense for this to be configurable though - we'll look at adding support for this ❤️