Add a quiet mode

I'd love a flag that disables printing the following lines:
```
Starting session with SessionId: <user>-<session>.
Exiting session with sessionId: <user>-<session>.
```

This was originally requested here:
https://github.com/aws/session-manager-plugin/pull/20#discussion_r1043911507

So I'm just opening a new issue to track this specifically.