Return useful error when encountering unknown kafka API key

[rukai]

We were loading the api key twice.
The first time we panicked if it was an unknown key.
And then the second time we gracefully returned an error.
This is a bit silly since we never hit the graceful error and always panic.

This PR fixes this so that we only load the api_key once and we perform graceful error handling that one time.

[github-actions]

1 benchmark regressed. 0 benchmark improved. Please check the benchmark workflow logs for full details: https://github.com/shotover/shotover-proxy/actions/runs/7895482233

                        Change within noise threshold.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) low mild
kafka_codec/encode_request_produce
                        time:   [1.1065 µs 1.1334 µs 1.1577 µs]
                        change: [+27.992% +34.195% +42.213%] (p = 0.00 < 0.05)
                        Performance has regressed.

[github-actions]

0 benchmark regressed. 1 benchmark improved. Please check the benchmark workflow logs for full details: https://github.com/shotover/shotover-proxy/actions/runs/7900968733

  2 (2.00%) low mild
  3 (3.00%) high mild
  1 (1.00%) high severe
kafka_codec/encode_request_produce
                        time:   [656.46 ns 669.73 ns 682.22 ns]
                        change: [-27.036% -25.362% -23.539%] (p = 0.00 < 0.05)
                        Performance has improved.