Record download times of smaller file sizes from partial completion times

In #25774 (moved) (and possibly elsewhere) we discussed options to derive download times of different file sizes than 50 KiB, 1 MiB, or 5 MiB from partial completion times.

As of now, OnionPerf (like Torperf) records timestamps whenever it completes deciles of the requested file size. Commit 13 on #25774 contains a sample graph of partial completion times derived from 1 MiB downloads.

If these partial completion timestamps are as accurate as timestamps for completing entire requests, which is something we should find out, let's consider switching OnionPerf's download model: it could download a single file size, 1 MiB or 5 MiB depending on available bandwidth, and record timestamps for 50 KiB, 100 KiB, 200 KiB, 1 MiB, 2 MiB, 5 MiB, and so on.

This isn't urgent. We decided on #25774 (moved) that development and deployment of this new feature are out of scope for the current roadmap until Mexico. Doesn't hurt to have this ticket, though, and maybe start discussing whether this even makes sense.