Add RTT to Sum when using JSON

# Context

* Version of iperf3:
all

* Operating system (and distribution, if any):
all

# Enhancement Request

* Current behavior
Currently the `RTT` stats are not shown in the Sum Object
```
"streams": [{
"socket": 6,
"start": 1.001069,
"end": 2.000895,
"seconds": 0.99982601404190063,
"bytes": 11927552,
"bits_per_second": 95437020.7014849,
"retransmits": 15,
"snd_cwnd": 139152,
"snd_wnd": 3142272,
"rtt": 13505,
"rttvar": 994,
"pmtu": 1422,
"omitted": false,
"sender": true
}],
"sum": {
"start": 1.001069,
"end": 2.000895,
"seconds": 0.99982601404190063,
"bytes": 11927552,
"bits_per_second": 95437020.7014849,
"retransmits": 15,
"omitted": false,
"sender": true
}
```

* Desired behavior
Display the RTT stats for each interval also in the sum object

```
"streams": [{
"socket": 6,
"start": 1.001069,
"end": 2.000895,
"seconds": 0.99982601404190063,
"bytes": 11927552,
"bits_per_second": 95437020.7014849,
"retransmits": 15,
"snd_cwnd": 139152,
"snd_wnd": 3142272,
"rtt": 13505,
"rttvar": 994,
"pmtu": 1422,
"omitted": false,
"sender": true
}],
"sum": {
"start": 1.001069,
"end": 2.000895,
"seconds": 0.99982601404190063,
"bytes": 11927552,
"bits_per_second": 95437020.7014849,
"retransmits": 15,
"omitted": false,
"rtt": 13505,
"rttvar": 994,
"sender": true
}
```

* Implementation notes
I'm curious why this hasn't been implemented yet. Is it because the mean RTT for the streams is considered useless, or has it simply been overlooked? I can handle the implementation, but I wanted to check before getting started.
For multiple streams i would choose mean.