Support "http_hidden_proxy" ENV var for hidden service only proxy (#18427)

* Support "http_hidden_proxy" ENV var for hidden service only proxy

* Fallback to http_proxy if http_hidden_proxy is not set

config/initializers/http_client_proxy.rb

@@ -18,5 +18,22 @@ Rails.application.configure do
     }.compact
   end
 
+  if ENV['http_hidden_proxy'].present?
+    proxy = URI.parse(ENV['http_hidden_proxy'])
+
+    raise "Unsupported proxy type: #{proxy.scheme}" unless %w(http https).include? proxy.scheme
+    raise "No proxy host" unless proxy.host
+
+    host = proxy.host
+    host = host[1...-1] if host[0] == '[' # for IPv6 address
+
+    config.x.http_client_hidden_proxy[:proxy] = {
+      proxy_address: host,
+      proxy_port: proxy.port,
+      proxy_username: proxy.user,
+      proxy_password: proxy.password,
+    }.compact
+  end
+
   config.x.access_to_hidden_service = ENV['ALLOW_ACCESS_TO_HIDDEN_SERVICE'] == 'true'
 end