Fix access token leak to logs from proxyagent (#13855)

This can happen specifically with an application service `/transactions/10722?access_token=leaked` request

Fix #13010

---

Saw an example leak in #13423 (comment)

```
2022-08-04 14:47:57,925 - synapse.http.client - 401 - DEBUG - as-sender-signal-1 - Sending request PUT http://localhost:29328/transactions/10722?access_token=<redacted>
2022-08-04 14:47:57,926 - synapse.http.proxyagent - 223 - DEBUG - as-sender-signal-1 - Requesting b'http://localhost:29328/transactions/10722?access_token=leaked' via <HostnameEndpoint localhost:29328>
```

changelog.d/13855.bugfix

@@ -0,0 +1 @@
+Fix access token leak to logs from proxy agent.

synapse/http/proxyagent.py

@@ -36,6 +36,7 @@
 from twisted.web.http_headers import Headers
 from twisted.web.iweb import IAgent, IBodyProducer, IPolicyForHTTPS
 
+from synapse.http import redact_uri
 from synapse.http.connectproxyclient import HTTPConnectProxyEndpoint, ProxyCredentials
 from synapse.types import ISynapseReactor
 
@@ -220,7 +221,11 @@ def request(
                 self._reactor, parsed_uri.host, parsed_uri.port, **self._endpoint_kwargs
             )
 
-        logger.debug("Requesting %s via %s", uri, endpoint)
+        logger.debug(
+            "Requesting %s via %s",
+            redact_uri(uri.decode("ascii", errors="replace")),
+            endpoint,
+        )
 
         if parsed_uri.scheme == b"https":
             tls_connection_creator = self._policy_for_https.creatorForNetloc(