Expose filter to allow injecting S3 StreamWrapper cache instance

[lukaspawlik]

Expose WordPress filter to allow other plugins to easily inject \Aws\S3\StreamWrapper cache instance.

This filter would make easy integration of solutions like https://github.com/lukaspawlik/s3-uploads-cache without doing hacks of re-registering StreamWrapper in your plugin.

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[joehoyle]

Thanks @lukaspawlik, this looks valuable. Since #475, the interface has slightly changed here, so this needs refreshing. Could you handle? Thanks!