Support deleting media from non-local storage providers

[nexy7574]

Adds a new method, delete to the StorageProvider abstract base class. Also implements it for FileStorageProviderBackend. Downstream this is used in matrix-org/synapse-s3-storage-provider#153

This change will allow external storage providers, such as synapse-s3-storage-provider, to delete media from their stores when media is removed. Previously, only media in the local provider would be removed, which means that deployments that stored their media on S3 would end up with dangling files, wasting storage.

Discussed in #synapse-dev:matrix.org

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