[DataStore] Refactor cloud data stores for better code reuse

Refactor the cloud data stores so that we can reuse code between them.

Currently oak-blob-cloud exists for S3DataStore and oak-blob-cloud-azure for AzureDataStore.  There's no common codebase between them, other than oak-blob-plugins which seems a bit too broad for sharing code between the cloud data stores.

The proposal is to repurpose oak-blob-cloud into common code for all cloud data stores, create oak-blob-cloud-s3 for S3DataStore, and then move what common code that can be shared between the two into the new oak-blob-cloud module.

This also would allow us to share testing code, which could be important especially for creating a cloud storage mock that will enable CI testing on the cloud data stores.