Support for routing outbound HTTP requests via a proxy (#6239)

The `http_proxy` and `HTTPS_PROXY` env vars can be set to a `host[:port]` value which should point to a proxy.

The address of the proxy should be excluded from IP blacklists such as the `url_preview_ip_range_blacklist`.

The proxy will then be used for
 * push
 * url previews
 * phone-home stats
 * recaptcha validation
 * CAS auth validation

It will *not* be used for:
 * Application Services
 * Identity servers
 * Outbound federation
 * In worker configurations, connections from workers to masters

Fixes #4198.

synapse/server.py

@@ -23,6 +23,7 @@
 # Imports required for the default HomeServer() implementation
 import abc
 import logging
+import os
 
 from twisted.enterprise import adbapi
 from twisted.mail.smtp import sendmail
@@ -168,6 +169,7 @@ class HomeServer(object):
         "filtering",
         "http_client_context_factory",
         "simple_http_client",
+        "proxied_http_client",
         "media_repository",
         "media_repository_resource",
         "federation_transport_client",
@@ -311,6 +313,13 @@ class HomeServer(object):
     def build_simple_http_client(self):
         return SimpleHttpClient(self)
 
+    def build_proxied_http_client(self):
+        return SimpleHttpClient(
+            self,
+            http_proxy=os.getenv("http_proxy"),
+            https_proxy=os.getenv("HTTPS_PROXY"),
+        )
+
     def build_room_creation_handler(self):
         return RoomCreationHandler(self)