Fix TLS when using the ProxyAgent

Same fix as #15746

Thanks to @realtyem for pointing it out!

`creatorForNetloc(...)` doesn't come with typing and expects `host`
to be `bytes` instead of a `str`.

`ProxyAgent` was introduced with the federation outbound proxy:
#15773

synapse/http/proxyagent.py

@@ -152,7 +152,7 @@ def __init__(
 
                 if federation_proxy.tls:
                     tls_connection_creator = self._policy_for_https.creatorForNetloc(
-                        federation_proxy.host,
+                        federation_proxy.host.encode("utf-8"),
                         federation_proxy.port,
                     )
                     endpoint = wrapClientTLS(tls_connection_creator, endpoint)