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

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

Fix https://github.com/matrix-org/synapse/issues/13010

---

Saw an example leak in https://github.com/matrix-org/synapse/issues/13423#issuecomment-1205348482

```
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>
```