[wip] Enable fseek for files in S3 storage

[ahti]

This allows nextcloud to use fseek when using a S3 storage backend, which enables range requests and thus video streaming including fast seeking, and other applications.

#8275 should be fixed by this.

The implementation is a new stream wrapper around the normal http stream. When fseek is used, a new stream is opened with a range header to begin at the right offset.

I'm not really a PHP developer, so there might be bugs, and there is no proper error handling when the storage backend doesn't reply with a content-range header (although I would expect S3 backends to support range requests properly), and probably in some other places, too.

I have not investigated this, but maybe this approach could also work for the other object store backends?

[firemdkfighter]

On version 17.0.1.1 don't work.

[kesselb]

Thanks 👍

Removed #14617 from the list because the issue is about SMB.

[SimplyCorbett]

This does not work for me on 18 RC2.

[icewind1991]

Thanks for the great work @ahti, very nice solution.

I've adjusted the stream logic to be generic over the http source to make it easy to use for other http based storage backends.

Since I can't push to your fork (and thus this PR), I've opened a new PR at #20033 and will close this one