Add missing WebIdentityTokenFileCredentialsProvider for S3ClientBuilder to allow use within EKS IRSA scenario

[scottaubrey]

Related to https://github.com/cantaloupe-project/cantaloupe/issues/637

I'm not a Java developer, so I'm not sure if this is right in any way, other than we do build with this patch and run it in production successfully. I welcome anything I can do to make this acceptable.

[nlisgo]

This looks like a great addition. Thanks for the contribution!