Extend OpenSubsonic support with Transcode Offset extension

TechKrzysiekrequested to merge
techkrzysiek/funkwhale:develop into develop

Hi, This merge request is adding Transcode Offset (transcodeOffset) extension from OpenSubsonic to Funkwhale.

https://opensubsonic.netlify.app/docs/extensions/transcodeoffset/

Tested with Symfonium app on Android.

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