Add initial-inflight-factor option to vespa-feed-client to mitigate the slow start

[yjtakamabe]

Is your feature request related to a problem? Please describe.

The feed client experiences a slow start because the default initial target inflight value is too low in our environment, causing a 2-3 minute delay in the time it takes to reach optimal throughput.

Describe the solution you'd like

I would like an option called "initial-inflight-factor" that allows us to configure a higher initial target inflight.
This would help the feed client ramp up more quickly and avoid performance bottlenecks at startup.

    --initial-inflight-factor <arg>       Multiplier for minInflight to
                                          determine the initial
                                          targetInflight

$ vespa-feed-client \
  --initial-inflight-factor 64 \
  --file /path/to/json/file \
  --endpoint https://container-endpoint:443/

Describe alternatives you've considered

One alternative we considered was setting the number of "connections" to 512 in order to mitigate the slow start.
However, this approach consumes significant machine resources.

$ vespa-feed-client \
  --connections 512 \
  --file /path/to/json/file \
  --endpoint https://container-endpoint:443/

Additional context

In our environment, we set the initial-inflight-factor to 64.