Add seekableTimeRanages didChange to ModernAVPlayerDelegate

[yaroslavlvov]

Would be nice to have a callback that indicates seekableTimeRanages changed for the current item, would be useful to update min and max for seek bar accordingly, instead of relying on asset duration. Super useful for lives which have sliding DVR window and I need to display UI accordingly

Proposed solution: extend current delegate api with func

    func modernAVPlayer(_ player: ModernAVPlayer, didSeekableTimeRangesChange range: (start: Double, end: Double))