Add S3 file storage

[ellmetha]

Description

Presently, Marten only provide a simple "file system" storage implementation allowing to interact with files located in a folder within the local file system. This works fine for relatively simple applications, but it should be possible to leverage file storages targeting commonly used cloud storage solutions.

In this light, let's introduce a new Marten::Core::Storage::S3 file storage implementation allowing to manipulate files within an S3 bucket.

Requirements

• The implemented storage should leverage the awscr-s3 shard
• It should be possible to define if files managed by such storage are public or not
• The use of additional upload options should be permitted