Terminal settings not restored after aws ecs execute-command

Hi, I hope I've found the right place for this bug.

After running `aws ecs execute-command --interactive`, I loose handling of special characters for erase, werase, kill, eof, etc by the terminal. Running `stty icanon` is sufficient to get things working again.

I am using version 1.2.295.0 on linux.

I found it necessary to run a shell to reproduce this. Executing /bin/true did not reproduce the issue.

```bash
:; aws ecs execute-command --interactive ... --command /bin/bash

The Session Manager plugin was installed successfully. Use the AWS CLI to start a session.

Starting session with SessionId: ecs-execute-command-0ffda0a0ffda0a0ff
root@ip-1-2-3-4# exit

Exiting session with sessionId: ecs-execute-command-0ffda0a0ffda0a0ff.

:; cat # observe erase and eof not being processed, interrupt still works
^?^?^?^D^C
:; stty icanon
:; cat # observe handlng of special characters is restored
:;
```

The issue in this case was the `icanon` flag not being reset to it's previous state, but it's a good idea to capture the existing termios settings and restore them before exiting.