Switch from httpclient to faraday

[nudded]

The latest release of http client gem is about 5 years ago. They are still shipping an embedded certificate trust store. We recently bumped into issues with this, as a cert renewed but it was no longer validated by http_client, but any other sane HTTP library would use the OS certs and could actually validate this.

This MR moves away from http_client gem and adapts Faraday and middleware. I understand this is a breaking change as the class passed in the http_config block is now a Faraday::Connection rather than a HTTPClient.

[nudded]

@nov Did you have time to check this?

[nov]

sorry for my late response.
the most important part of httpclient was logging, and faraday has similar feature.
let's try to move faraday now.