Websocket Keepalive -- PingTimeInterval

The websocket keepalives are set fairly high at 5 minutes.
https://github.com/aws/session-manager-plugin/blob/1c9b600eca0b8b7d6e1e0b09dcdd7a78551582f5/src/config/config.go#L38

I am a large customer whose developers operate behind a busy corporate proxy which aggressively closes idle connections.

Would you be able to make this valuable configurable? Even if it was via environment variable that is fine. I need to lower this to around 15-30 seconds probably.

```
2021-10-13 00:48:01 DEBUG Closing websocket connection to:%!(EXTRA string=1.2.3.4:1234)
2021-10-13 00:48:01 ERROR Failed to close websocket: tls: failed to send closeNotify alert (but connection was closed anyway): write tcp 4.3.2.1:50638->1.2.3.4:1234: write: broken pipe
2021-10-13 00:48:01 DEBUG Closing datachannel failed with error: tls: failed to send closeNotify alert (but connection was closed anyway): write tcp 4.3.2.1:50638->1.2.3.4:1234: write: broken pipe
2021-10-13 00:48:01 INFO Opening websocket connection to: %!(EXTRA string=wss://ssmmessages.us-east-1.amazonaws.com/v1/data-channel/blah@blah.com-12345678912345678?role=publish_subscribe)
2021-10-13 00:48:01 DEBUG Resend stream data message: 1
2021-10-13 00:48:01 ERROR Unable to send stream data message: Can't send message: Connection is closed.
2021-10-13 00:48:01 DEBUG Resend stream data message: 1
2021-10-13 00:48:01 ERROR Unable to send stream data message: Can't send message: Connection is closed.
......
2021-10-13 00:48:01 DEBUG Resend stream data message: 1
2021-10-13 00:48:01 DEBUG Resend stream data message: 2
2021-10-13 00:48:01 DEBUG Resend stream data message: 3
2021-10-13 00:48:01 DEBUG Resend stream data message: 4
2021-10-13 00:48:01 DEBUG Resend stream data message: 5
2021-10-13 00:48:01 DEBUG Resend stream data message: 6
```