Feature request: Allow configurable delay between requests

[emilyst]

1. What's the feature?

   This is sort of a bug, sort of a feature.

   Problem: Because ddns-updater issues all its requests as quickly as possible, services with rate limits (such as Porkbun) will reject requests if you have a non-trivial number of records to update.

   Proposed solution: Allow for configuring a brief delay between each request, in order to avoid being rate-limited.

2. Extra information?

   Right now, I can't actually use ddns-updater until this kind of capability exists, as nice as I find this software to be! I happen to use Porkbun to provide my DNS, and their reverse proxy returns 502 errors if you make requests too quickly.

   Maybe also nice to have, but not necessary for my particular case:

   • Allow for some jitter, or exponential backoff, in the delay.
   • Detect rate-limiting responses or certain kinds of gateway errors and in those situations automatically introduce a delay.

Thanks in advance!