[session-manager-plugin] "aws ssm start-session" should print debug output to stderr instead of stdout

Confirm by changing [ ] to [x] below to ensure that it's a bug:
- [x] I've gone though the [User Guide](https://docs.aws.amazon.com/cli/latest/userguide/cli-chap-welcome.html) and the [API reference](https://docs.aws.amazon.com/cli/latest/reference/)
- [x] I've searched for [previous similar issues](https://github.com/aws/aws-cli/issues) and didn't find any solution

**Describe the bug**

`ssm start-session` should enable the user to pipe stdout to a file or other command.
Currently, it dumps debug output to stdout, which is not helpful.

**SDK version number**
Same behavior with v1 and v2.
`aws-cli/1.19.12 Python/3.8.5 Linux/4.19.128-microsoft-standard botocore/1.20.12`
`aws-cli/2.1.27 Python/3.7.3 Linux/4.19.128-microsoft-standard exe/x86_64.ubuntu.20 prompt/off`

`$ session-manager-plugin --version -> 1.2.54.0`

**Platform/OS/Hardware/Device**
Ubuntu 20.04.1 LTS (in WSL2)

**To Reproduce (observed behavior)**

```
$ aws ssm start-session --target i-0123456789abcde --document-name AWS-StartInteractiveCommand --parameters 'command=["echo 42"]' > 42.txt
(no output)

$ cat 42.txt

Starting session with SessionId: root-0001234567abcde
42

Exiting session with sessionId: root-0001234567abcde.
```

**Expected behavior**
```
$ aws ssm start-session --target i-0123456789abcde --document-name AWS-StartInteractiveCommand --parameters 'command=["echo 42"]' > 42.txt

Starting session with SessionId: root-0001234567abcde

Exiting session with sessionId: root-0001234567abcde.

$ cat 42.txt
42
```

**Logs/output**
n/a

**Additional context**
Use case: Run a command on the instance, and do some follow-up actions (local or remote) based on the output.

Unix commands generally print additional info to stderr.
Only the main output / payload should go to stdout.

I could find the text "Starting session" / "session with SessionId" neither in this repo, nor in the botocore repo.
-> I assume that this needs to be fixed in the `session-manager-plugin`, which seems not to be available on Github.

The report names no file or test and says the output comes from session-manager-plugin, outside this repository. Start by reproducing the aws ssm start-session command with stdout redirected and trace ownership of the plugin output; done means only 42 is in 42.txt while session messages appear on stderr.

Written by the indexing model from the issue text.