Allow plugins to access the default pipeline role

[graytaylor0]

Is your feature request related to a problem? Please describe. The data-prepper-config.yaml supports passing a default sts_role_arn. Sometimes, plugins are checking for this role specifically and still treating it as required. For example, the S3 source uses the sts_role_arn in some cases to determine the bucket owner.

Describe the solution you'd like Add support for plugins to access the default pipeline role via AwsCredentialsSupplier, similar to how the default region is accessible.

Describe alternatives you've considered (Optional) A clear and concise description of any alternative solutions or features you've considered.

Additional context Add any other context or screenshots about the feature request here.