feature request: make client report after specific number of bytes transferred

# Context

* Version of iperf3: 3.13

* Hardware: x86

* Operating system (and distribution, if any): Debian 12

# Enhancement Request

* Current behavior

it is currently possible to generate a report every number of seconds using `-i` flag, however, there is no way to report after a specific number of bytes are transferred.

* Desired behavior

provide a new flag to report every number of bytes, for example:

```shell
$ iperf3 --interval-bytes=1M
```

would report after each 1MB was transferred regardless of how much time has elapsed.

it's fine if `-i 0` also needs to be passed along to disable time based intervals

* Implementation notes

i'm using this to generate a sound each time a number of bytes is successfully sent on my phone. this helps me search for reception while traveling in remote areas. more frequent sound means higher bandwidth.

i tried to do this using repeated invocations to `iperf3`, but the startup time (connecting initial server connection/handshake) ends up being the majority of the time and puts an upper bound on the frequency of notifications.

No files or tests are named. Start by locating the client-side `-i` interval handling, byte counters, and command-line option parsing; compare how time-based reports are generated. Done means a new byte-based interval option reports at the requested transfer amount, works with the proposed `-i 0` combination, and preserves existing interval behavior.

Written by the indexing model from the issue text.