Delete Media on Media storage providers

[Alkl58]

Description:

Currently the implementation for the media_storage_providers is only able to upload and download - with options whether or not to store locally uploaded files and / or to store remote media (media cache), however we can only delete local media files.

When looking at the code, we only delete local files:

• delete_old_remote_media
• _remove_local_media_from_disk

There are probably some questions which have to be answered first:

• Should the deletion be optional? (e.g. option in config)
• Would the implementation break existing storage providers?
• Is this "feature" even something we want?

I am personally running a server with the synapse-s3-storage-provider and noticed relatively late, that it's basically append only, meaning the s3 storage will continuously grow without being able to delete old/unused media. (Related Github Issue matrix-org/synapse-s3-storage-provider#96)

I may or may not be able to contribute, I would need to setup a dev environment for that.