Introduce new Vary strategy

[skryukov]

The initial problem: we use GitHub API and to save on API limits we want to use HTTP caching, but with the current Faraday::HttpCache::Storage implementation (one cache key per URL) caching of GitHub API endpoints like https://api.github.com/user is impossible since for every request we are fetching an array of all requests for all our users. Limiting the number of cached entries per key from #121 is also not an option in this case.

So, the only way to cache GitHub API for us is implementing different storage strategy. That is what this PR is about.

This PR Introduced a new strategy option to support different cache storage strategies:

• The original strategy was moved from Faraday::HttpCache::Strage to Faraday::HttpCache::Strategies::ByUrl.
• The new Faraday::HttpCache::Strategies::ByVary strategy uses headers from Vary header to generate cache keys. It also uses the index with Vary headers mapped to the request URL.
• Faraday::HttpCache::Strage class was deprecated.

Related #112
Part of #127