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.

tests/push/test_http.py

@@ -50,7 +50,7 @@ class HTTPPusherTests(HomeserverTestCase):
         config = self.default_config()
         config["start_pushers"] = True
 
-        hs = self.setup_test_homeserver(config=config, simple_http_client=m)
+        hs = self.setup_test_homeserver(config=config, proxied_http_client=m)
 
         return hs