[Feature Request] Time series CSV / JSON output

Hi, thank you for developing this software - it's fantastic.

Would it be possible to add in a time-series/output that counts the number of requests, grouped/counted at a specified unit of time and emits that as CSV?

This could be used to generate a line graph or histogram of requests per second over time.

Something like this:

```text
$ oha --time-series 1s > output.csv
$ cat output.csv

time(s),count
0, 100
1, 120
2, 110
3, 100
4, 90
5, 100
```

This is helpful in demonstrating the consistency in the delivery of requests. Take for example this run:

https://github.com/user-attachments/assets/01621ddb-f204-4750-ae38-9e0d031f4e45

The "average time" is not helpful in describing the service experience.