Add direct AWS download method

[tanaes]

Description of feature

SRA is currently mirroring their files on AWS, so it seems like adding an AWS-direct method might be a useful method, particularly for anyone launching workflows in the cloud.

In my own little pipeline, I've set it up like this:

https://github.com/tanaes/nf-reads-profiler/blob/d5a3ec72cb8aeeff7bd0df1db03e3f583ea21842/modules/data_handling.nf#L1-L24

I just give it a samplesheet with an optional SRA column (see schema here) and, if it finds a valid SRA ID for that sample, it will use aws s3 cp to download the file and feed to fasterq_dump.

This method has been incredibly fast and reliable for me! It would be great to see it in fetchngs.