Add support for AWS S3 as a media library

**Is your feature request related to a problem? Please describe.**
Currently, there is no built-in support for using AWS S3 as a media library in Decap CMS. This makes it difficult for users who rely on S3 for their media storage to integrate it seamlessly with the CMS. Manually uploading to S3 and pasting the URL is a possible workaround, but it is not an efficient workflow for content editors.

**Describe the solution you'd like**
I would like to request the addition of native support for AWS S3 as a media library. This would involve:

1. A new configuration option in `config.yml` to specify S3 as the media library.
2. Settings for S3 bucket details, such as bucket name, region, and access credentials (using best practices for security, like IAM roles or access keys).
3. The ability to browse, upload, and delete files in the specified S3 bucket directly from the Decap CMS media library interface.

**Describe alternatives you've considered**
I have considered using other media libraries or manually managing S3 assets, but these solutions lack the convenience and integration of a native S3 media library. A built-in solution would greatly improve the user experience for a significant number of users.

**Additional context**
Many projects already use AWS S3 for its scalability, reliability, and cost-effectiveness. Adding S3 support would make Decap CMS an even more attractive option for developers and content creators who use the AWS ecosystem.