backend/s3: Add multiple credentials and config files parameters

[gdavison]

The AWS SDK for Go v2 allows passing multiple shared credentials and shared config files. Note that the corresponding environment variables, AWS_SHARED_CREDENTIALS_FILE and AWS_CONFIG_FILE accept single files only.

1. Document support for both environment variables
2. Add parameter shared_credentials_files
3. Add parameter shared_config_files
4. Deprecate shared_credentials_file

Depends on #30443

[kmoe]

Thanks, this looks reasonable.

[github-actions]

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