Support for connecting to private S3 buckets

[MatthewTe]

Been using Terracotta for a while in personal projects and wanted to connect and serve GeoTiffs from a self hosted minio object store that requires authentication.

I'm not sure if there is already a way to do this in Terracotta but this issue Issue #1293 and this documentation about the rasterio VFS indicates that it should be straightforward to pass an AWSSession based on credentials provided via the TerracottaSettings to the rasterio.Env context that allows the RasterStore to read rasters from S3.

I've got a feature branch with an implementation the seems to work - not formatted/linted but just wanted to put this out there to see if this feature already exists or if there is a better way to implement this. Let me know if I should just create a PR for this.

[MatthewTe]

Created a draft PR with the minimal changes necessary to get the functionality working #354

[dionhaefner]

That looks useful, thanks. Will weigh in on that PR so we can iterate together.