Add json-stream formatting as additional parameter to standard json ouput

[cynak]

Added a couple new suboption parameters in the use -J json output format.

With use of the -J parameter a new argument --json-output-stream can be used to output a single stream output utilizing JSONStream_Output.

With use of the --json-stream argument a new --json-stream-sum-only which allows a core functionality of -J of not showing interval data and providing only a final output. --json-stream-sum-only displays only the final output that would show in --json-stream

Arguments can be daisy chained along with --json-stream in example
iperf3 -c 127.0.0.1 --json-stream --json-stream-sum-only --json-stream-full-output --json-output-stream

{"event":"end","data":{"streams":[{"sender":{"socket":5,"start":0,"end":10.000368,"seconds":10.000368,"bytes":121313689600,"bits_per_second":97047380336.403625,"retransmits":0,"reorder":0,"max_snd_cwnd":3405116,"max_snd_wnd":6192000,"max_rtt":22,"min_rtt":17,"mean_rtt":18,"sender":true},"receiver":...

`

You are under no obligation whatsoever to provide any bug fixes, patches, or
upgrades to the features, functionality or performance of the source code
("Enhancements") to anyone; however, if you choose to make your Enhancements
available either publicly, or directly to Lawrence Berkeley National
Laboratory, without imposing a separate written license agreement for such
Enhancements, then you hereby grant the following license: a non-exclusive,
royalty-free perpetual license to install, use, modify, prepare derivative
works, incorporate into other computer software, distribute, and sublicense
such enhancements or derivative works thereof, in binary and source code form.

The complete iperf3 license is available in the LICENSE file in the
top directory of the iperf3 source tree.

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

• Issues fixed (if any):

• Brief description of code changes (suitable for use as a commit message):