[extractor/stageplus] Add extractor

[bashonly]

stage-plus.com is a subscription-only site that streams live/VOD concerts and albums, mostly of classical music.

This PR only adds support a VOD concerts, not live concerts or albums.

Each VOD concert is a playlist made up of multiple works (videos). I opted not to make them multi_video playlists, because each work is then broken down into multiple movements (chapters).

There is a huge graphQL request body, but most of the metadata being requested is either useful or potentially useful (if/when livestream/album extractors are added). It seemed more efficient to leave it intact than to try and carefully perform surgery on it, only to maybe have to restore parts later.

Closes #6806

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[pukkandan]

There is a huge graphQL request body, but most of the metadata being requested is either useful or potentially useful (if/when livestream/album extractors are added). It seemed more efficient to leave it intact than to try and carefully perform surgery on it, only to maybe have to restore parts later.

Add a #TODO that it need to be cleaned up after live is impl