feat: show rates msgs and bytes per sec using spc

[wallyqs]

Add event to toggle showing msgs and bytes per sec when pressing space:

NATS server version 2.9.16-RC.5 (uptime: 1d13h59m50s)
Server: nats-burn-0
  ID:   NCKKALSWW7K35JNVZ2E32EC3NTTGZBYYDVBGPLFE7BQ32KKJITO5HVEV
  Load: CPU:  302.0%  Memory: 3.2G  Slow Consumers: 0
  In:   Msgs: 2.5G  Bytes: 1286.7G  Msgs/Sec: 126043.2  Bytes/Sec: 52.2M
  Out:  Msgs: 2.5G  Bytes: 1151.6G  Msgs/Sec: 121019.3  Bytes/Sec: 64.4M 

Connections Polled: 6
  HOST                CID    NAME                            SUBS    PENDING     MSGS_TO     MSGS_FROM   BYTES_TO    BYTES_FROM  LANG     VERSION  UPTIME   LAST_ACTIVITY
  10.8.3.246:34910    15999                                  5002    0           58.6K       87.9K       49.2M       10.1M       go       1.23.0   3h56m14s  2023-04-06 15:46:58.269915825 +00
  10.8.3.247:32854    19391  nats:burn-in:streams-producer   1       0           538         538         18.5K       1.1M        go       1.23.0   3h55m12s  2023-04-06 15:46:58.25628237 +000
  10.8.3.249:57708    19392  nats:burn-in:streams-producer   1       0           535         535         18.5K       1.0M        go       1.23.0   3h55m11s  2023-04-06 15:46:58.257240737 +00
  10.8.3.248:58338    19394  nats:burn-in:streams-producer   1       0           527         527         18.2K       1.0M        go       1.23.0   3h55m11s  2023-04-06 15:46:58.256263368 +00
  10.8.3.250:33976    19395  nats:burn-in:streams-producer   1       0           536         535         18.5K       1.0M        go       1.23.0   3h55m11s  2023-04-06 15:46:58.272797262 +00

Command          Description
     
...
space            Toggle displaying rates per second in connections.

Press any key to continue...

[derekcollison]

LGTM