This will cache any GitHub API responses with correct Cache-Control header.
`gregjones/httpcache` has been chosen as a library to implement this feature, as it is as recommended in `go-github`'s documentation:
https://github.com/google/go-github#conditional-requests
`gregjones/httpcache` supports a number of cache backends like `diskcache`, `s3cache`, and so on:
https://github.com/gregjones/httpcache#cache-backends
We stick to the built-in in-memory cache as a starter. Probably this will never becomes an issue as long as various HTTP responses for all the GitHub API calls that ARC makes, list-runners, list-workflow-jobs, list-runner-groups, etc., doesn't overflow the in-memory cache.
`httpcache` has an known unfixed issue that it doesn't update cache on chunked responses. But we assume that the APIs that we call doesn't use chunked responses. See #1503 for more information on that.
Ref #920
4c53e3aa75