perf(ext/fetch): speed up resp.clone()

[lucacasonato]

main (before)

Running 10s test @ http://localhost:8000
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   123.96us   57.25us   3.64ms   96.84%
    Req/Sec    41.15k     2.11k   42.76k    95.05%
  827087 requests in 10.10s, 125.41MB read
Requests/sec:  81895.64
Transfer/sec:     12.42MB

this branch

Running 10s test @ http://localhost:8000
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    66.91us   17.71us   2.03ms   95.91%
    Req/Sec    74.43k     2.52k   76.71k    92.57%
  1496402 requests in 10.10s, 199.79MB read
Requests/sec: 148170.16
Transfer/sec:     19.78MB

Fixes #24766

[littledivy]

LGTM!