Add Functionality to Add PDFs from Amazon S3

[nipunTharuksha]

Currently, the only way to add PDFs is through the function addPDF() that can only be linked to pdfs that already exist on the local, and integration or inclusion to the plugin to pull pdfs from Amazon S3 buckets before including them would be very useful to users. This addition would provide more flexibility and better user experience, helping users merge from pdfs in the cloud without necessarily downloading them locally.
For us to achieve this, we propose to add another function addPDFFromS3() to the plugin that will factor the file’s path on S3, and all the other parameters familiar with the existing addPDF() function.

[gotDaijobu]

Useful, tested and working PR. Thanks.

[gotDaijobu]

Had a few challenge in production with Nipun's solution.

I worked on an alternative, based on Nipun's work, that used temp files locally instead of temp urls.

It's available Here