Add support for AWS anonymous access

[pditommaso]

Summary

In some use cases, it's needed to be public data hosted on S3 bucket, however the S3 client for Nextflow asks the S3 credentials even if not really needed.

This can be a limiting factory in some contexts. The goal of this issue is to prevent the need of using AWS creds to access public data on S3.

This should be achievable using the AnonymousAWSCredential when creating the AWS client.

This option could be provided by specifying the following setting in the nextflow config

aws.client.anonymous = true

[pditommaso]

Solved by ae7ba35