support sso-session in aws credentials config, for IdC (IAM Identity Center) profiles

Something like:

```json
"customizations": {
"vscode": {
"settings": {
"aws.sso.startUrl": "<...>",
"aws.sso.region": "<...>",
}
}
}
```

Start by locating the AWS credentials configuration handling and the existing VS Code customization settings for SSO profiles. Trace how an sso-session profile is represented and determine where the aws.sso.startUrl and aws.sso.region settings should be accepted; done means IdC profiles can be configured without unsupported-setting errors.

Written by the indexing model from the issue text.