Transform `MediaStorage.local_media_directory` into an optional storage provider

[vfosnar]

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.