Add POST object put benchmark

[klauspost]

Add --post parameter to warp put, which will use Form POST Object API.

Example

λ warp put -tls -post -duration=15s -obj.size=1KB
warp: Benchmark data written to "warp-put-2024-09-11[115431]-4IRH.csv.zst"

----------------------------------------
Operation: POST. Concurrency: 20
* Average: 0.12 MiB/s, 122.12 obj/s

Throughput, split into 13 x 1s:
 * Fastest: 127.5KiB/s, 130.61 obj/s
 * 50% Median: 120.9KiB/s, 123.78 obj/s
 * Slowest: 102.0KiB/s, 104.48 obj/s
warp: Cleanup Done.