Add a quiet mode

I'd love a flag that disables printing the following lines:
```
Starting session with SessionId: -.
Exiting session with sessionId: -.
```

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.

Start with the linked pull-request discussion, then locate where the session-start and session-exit lines are printed and where command flags are defined. Done means a quiet-mode flag suppresses both specified lines while session behavior remains unchanged; check any existing command tests around these entry points.

Written by the indexing model from the issue text.