The assumption that a download URL is always HTTP is no longer correct. Add support for downloading via the AWS SDK when the URL protocol is s3://, keeping the existing fetch path for http(s)://.

• Region and credentials are read from the environment, not passed explicitly.
• When credentials are absent (e.g. a local env), fail with an actionable message mentioning lack of access.

Code: https://github.com/mongodb-js/devtools-shared/blob/main/packages/mongodb-downloader/src/index.ts#L168-L253