Add callback function to get JSON Output strings

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master

• Issues fixed (if any): How to use the iperf library to obtain bandwidth information in real-time at intervals? #1711

• Brief description of code changes (suitable for use as a commit message):
  Add iperf_set_test_json_callback() that allows a user app to set a callback function to get the JSON output strings, instead of printing them to the output file.

[swlars]

Thanks for the pull request! This looks useful!

[bmah888]

Everything seems to work correctly. I have one question about the intended callback function...all of the other callbacks take a test argument. For those callbacks (such as on_new_stream, on_test_start, etc.), the test pointer gives all the information needed by the callback. The json_callback is different, in that it looks like it only takes a string containing a JSON rendered object. Do you think that having the test structure as well might be needed to give some context for working with the JSON? So then callback would take both a (iperf_test *) and a (char *)?

EDIT: I didn't need this change for the testing I had, but I'm just wondering if having a pointer to the test structure might be useful, say to refer to test parameters, etc.