Hard-coded short open timeout hurts when pull-through caching on high-latency ISP

### Okay, here's the situation. We're using gemstash to reduce the load on sparse internet this weekend at Ruby for Good. Our internet connection has some pretty high latency. gemstash's missing-in-cache-so-fetch-it activity would often run afoul of [the 2-second hard-coded timeout for gemstash to open a connection to the configured upstream](https://github.com/rubygems/gemstash/blob/v2.7.1/lib/gemstash/http_client.rb#L58). ### My parents went away on a week's vacation. I whittled the replication case down to the following run while on our sparse internet connection: ```ruby root@gemstash:/usr/src/app# irb irb(main):001:0> require 'faraday' => true irb(main):002:0> require "faraday_middleware" => true ``` Just enough [HTTPClient.for()](https://github.com/rubygems/gemstash/blob/v2.7.1/lib/gemstash/http_client.rb#L33-L37) … ```ruby irb(main):014:1* client = Faraday.new('https://rubygems.org') do |config| irb(main):015:1* config.use FaradayMiddleware::FollowRedirects irb(main):016:1* config.adapter :net_http irb(main):017:1* config.options.timeout = 10 # what we're using for the configurable :fetch_timeout irb(main):018:0> end => #<Faraday::Connection:0x0000007fa2fb2f80 ... ``` Just enough [client.get()](https://github.com/rubygems/gemstash/blob/v2.7.1/lib/gemstash/http_client.rb#L55-L60) … ```ruby irb(main):019:0> client.get('/gems/rails-7.1.3.3') { |req| req.options.open_timeout = 2 } /usr/local/lib/ruby/3.2.0/net/http.rb:1271:in `initialize': Failed to open TCP connection to rubygems.org:443 (execution expired) (Faraday::ConnectionFailed) … 😭 irb(main):020:0> client.get('/gems/rails-7.1.3.3') { |req| req.options.open_timeout = 10 } => #<Faraday::Response:0x0000007fa2879c50 … ✅ ``` ### They left the keys to the brand new Porsche So, I've [forked and hard-coded a larger timeout](https://github.com/rubygems/gemstash/compare/main...robbkidd:gemstash:49595be) for the moment. ### Would they mind? Would the gemstash project be up for [the open_timeout](https://github.com/rubygems/gemstash/blob/v2.7.1/lib/gemstash/http_client.rb#L58) becoming configurable like the fetch timeout? I'm happy to start a PR. … with some guidance on the nuances of the implementation. For my temporary hack, I first attempted to [naively reuse `:fetch_timeout`](https://github.com/rubygems/gemstash/commit/2dffd6a12a242189329425749e4ec8cef87ed1c3) and it became clear that the `gemstash_env` utility is currently only mixed in as a class method (`extend Gemstash::Env::Helper`), not instance (`include Gemstash::Env::Helper`). If this open_timeout becomes configurable, I don't know if you would prefer mixing in as instance methods, too, or passing the timeout as a new parameter to HTTPClient.new(), or some other means I haven't thought of.