Add "gateway benchmark-stream"(PRIME-651, PRIME-654, PRIME-655)

[vdavid]

This PR:

• adds the gateway benchmark-stream endpoint similar to gateway benchmark with the capability to test the actual code completion endpoints on SG and CG instances.
• adds the --use-special-header flag and optional request-level CSV output to the "benchmark gateway" command.

This is done for benchmarking purposes for these three issues:

• https://linear.app/sourcegraph/issue/PRIME-651/look-into-cloudflare-settings-for-gateway-vs-dotcom-streaming
• https://linear.app/sourcegraph/issue/PRIME-655/investigate-why-double-streaming-incurs-an-additional-123ms-of-latency
• https://linear.app/sourcegraph/issue/PRIME-654/investigate-why-the-fastpathnon-fastpath-diff-decreases-with-longer

See those issues for more context.

Test plan

Manual QA: I used the tool several times after the changes.

[camdencheek]

@vdavid is this ready for review now? And is Code Search who you want reviewing it?

[vdavid]

@camdencheek, very sorry, I have no idea how I got Code Search tagged on this one. Pls ignore.

[camdencheek]

Haha, no worries. I think we're auto-tagged on this repo since we're codeowners.

[vdavid]

@slimsag, I've added some more changes to this but it's roughly the same as when you reviewed it, plus I went another quick round on it, adding a bit more docs and the such. I'll merge it to avoid growing it further, but feel free to re-review it and ask for fixes.