Implement backoff in case of throttling by GitHub

Should we consider rate limiting here? Just curious about downtime and we have to send a ton of these, we could hit rate limits and need to throttle ourselves. Rather than failing completely we could back off (or check headers for the next time we can send).

This could be a follow up.

_Originally posted by @verbanicm in https://github.com/abcxyz/github-metrics-aggregator/pull/63#discussion_r1195836920_