SSO does not allow specifying the region

# Problem

`region` is not a parameter to the SSO module:
https://github.com/aws/language-servers/blob/17a6f6f864125c8cb2c5c936e66bf69e1eebf2ee/server/device-sso-auth-lsp/README.md

# Expected behavior

- Auto-discover the correct region based on a provided start url.
- Allow consumers to provide an explicit `region` parameter, as a fallback if auto-discover fails.