Simpler configuration of options (like code-server version)

I would like to automate the setup of codeserver, but I'd like to change the code-server version. Currently, both setup-codeserver and install-codeserver need to be edited to change this. This would be possible with something like `sed`, but a bit clunky.

I think either using cli arguments or environment variables would be a good solution. Environments variables would be a small change from:

```
CODE_SERVER_VERSION="4.5.2"
```
to
```
CODE_SERVER_VERSION="${CODE_SERVER_VERSION:-4.5.2}"
```

Then my installation script would be:
```
curl -LO https://github.com/aws-samples/amazon-sagemaker-codeserver/releases/download/v0.1.5/amazon-sagemaker-codeserver-0.1.5.tar.gz
tar -xvzf amazon-sagemaker-codeserver-0.1.5.tar.gz

cd amazon-sagemaker-codeserver/install-scripts/notebook-instances

chmod +x install-codeserver.sh
chmod +x setup-codeserver.sh
export CODE_SERVER_VERSION="4.13.0"
sudo ./install-codeserver.sh
sudo ./setup-codeserver.sh
```
Would this be possible to implement? I can propose a PR if it helps.