Implement a FileSystem backed by S3

[jereanon]

We need an S3 backed virtual file system for a few applications we've created over at Optact. This project seemed like a good opportunity to contribute to something that is already in use, rather than creating something of our own.

If this an interesting addition to the project, we'll create a draft PR soon.

• [ ] Implement AsyncFileSystem
• [ ] Implement FileSystem
• [ ] Make this implementation a feature with proper dependencies for S3 and existing async feature
• [ ] Figure out a sane way to create unit tests for this feature
• [ ] Update readme

[jereanon]

We decided to build something in house for our needs