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/handlers/ui_auth/checkers.py

@@ -81,7 +81,7 @@ class RecaptchaAuthChecker(UserInteractiveAuthChecker):
     def __init__(self, hs):
         super().__init__(hs)
         self._enabled = bool(hs.config.recaptcha_private_key)
-        self._http_client = hs.get_simple_http_client()
+        self._http_client = hs.get_proxied_http_client()
         self._url = hs.config.recaptcha_siteverify_api
         self._secret = hs.config.recaptcha_private_key