Attach files to email from S3

[geniusdibya]

Add a capability to the current Pepipost Ruby SDK to attach files to an email from an S3 bucket.

Example: A user might be generating all transactional receipts in PDF format in S3 and those need to be sent through emails.

[neeraj-joshi01]

@geniusdibya Can we do that as an attachment to the mails?