Transform MediaStorage.local_media_directory into an optional storage provider #16751
Description
This issue has been migrated from #16751.
Description:
To address #7140 and matrix-org/synapse-s3-storage-provider#28, we need reimplement MediaStorage.local_media_directory as a storage provider (MediaStorage.storage_providers). This will allow us to optionally disable local media storage in case we want to store data remotely, for example on S3.