Add support for AWS S3 filestorage via custom adapter

[timstallmann]

A client I'm working with wanted to migrate to S3, but I really did not want to lose Archive Repertory support, and we're still on Omeka Classic.

The built-in Zend S3 filesystem adapter, unfortunately, is stuck on v2 of Amazon's AWS request signing protocol, which means that the file move operation breaks. Since that's a key part of Archive Repertory functionality, enabling S3 here requires using the EHRI custom S3 filesystem adapter, which supports v4 request signing.

But then, that custom adapter doesn't have a built-in method to check for file existence, so I added one in a PR, which hopefully will be merged at some point. But for now, you'll need to use my fork of the plugin.

So far this approach seems to work well; I'll likely continue to update over the next few days as we keep testing it out in production.