Scrape Github repo index for metrics on singer-esque forks and repos

As noted here, it should be possible to automatically scrape a full index of repos: https://github.com/PyGithub/PyGithub/issues/723

According to the API docs, it should be possible to also capture the repo creation date, commits, etc. We might run into rate limiting but there's an optional "since date" in their API we could use for our replication key.

For team metrics and for presentations, would be a good metric to be able to track. Singer Hub could similarly auto-discover new forks in this way, except we would not want to auto-promote new taps - for obvious reasons.

Once the set of repos we need data from is identified, we could also scrape vitality metrics such as "last commit date", or "last PR merge", "average PR age", etc.