Include `username` in API logs

In the case of authenticated queries from API, include the username in the API logs for debugging purposes.

For example, API log when a user subscribes to a channel looks like this:
```
kernelci-api | INFO: 172.20.0.1:42984 - "POST /latest/subscribe/node HTTP/1.1" 200 OK
```

That should contain `username` of requesting user:
```
kernelci-api | INFO: 172.20.0.1:42984 - staging.kernelci.org "POST /latest/subscribe/node HTTP/1.1" 200 OK
```

Start at the API request logging path for authenticated queries, using POST /latest/subscribe/node as the concrete entry point. Verify that the completed log includes the requesting username while preserving the existing request details, and exercise that subscription request to confirm the output.

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