[S3] Update S3 bucket and the fetching secrets method

## 🎉 Description

There's a new S3 bucket for the builder server that uses CloudFront. This new mechanism will allow us to save money as cloudfront can cache the request more cost-effectively than serving content directly from S3.

Tasks:
- [ ] Update S3 bucket URL to: `https://builder-assetspack.decentraland.org/`
- [ ] Fetch the new credentials using `aws secrets manager`