New Blob Store API

A new blob store API has been introduced for storing blobs of data. By default, this data is stored in the file system in the data directory, but admins can configure XWiki to store the data on S3, which makes the data centrally available on clustered XWiki instances. Extensions should use this API as a replacement for permanent file storage. By choosing appropriate store names or customizing the storage location, it is possible to switch from plain file system access to the blob store API without any data migration. XWiki supports automatically migrating between the configured store and a migration store, allowing for automatic migration of the existing file system data to S3 when configured.