Cloud Storage backend alternatives for testing

[DiegoPino]

What?

AWS S3 testing right now uses min.io. It is a very capable S3 compatible server but in our production experience (and depending on the time-based release) some of the specs, in specific in our experience for multipart upload requirements can be a bit off-sync with the official AWS ones.

One alternative, if desired, would be to use https://github.com/localstack/localstack which is intended for developers to test all current AWS specs/APIs against services locally, which includes S3 storage too. In terms of running this. there would be not much difference, it is also dockerized. Might also allow to test things like #637 which can not be tested using just min.io or an official AWS account (paid)

For Azure Object Storage there is https://github.com/azure/azurite, which is an official Microsoft Azure local implementation of their blob storage and thus aligned with their specs. Also runs on docker.