Allow configurable qos

[rromanchuk]

This is a bit ugly, but PoC test to resolve priority inversion warnings

Thread Performance Checker: Thread running at QOS_CLASS_USER_INITIATED waiting on a lower QoS thread running at QOS_CLASS_DEFAULT. Investigate ways to avoid priority inversions
PID: 22258, TID: 26715793

This is probably more relevant in an async/await context.

Example reproduction where callee uses qos of .userInitiated

Task(priority: .userInitiated) {
   try? await model.process()
}

    @MainActor
    class ViewModel: ObservableObject {
        @Published var proccessedVideo: URL?
        init() {}
        
       func process() async throws {
            let nl = NextLevelSessionExporter(withAsset: mixComposition)
            // nl.videoComposition = videoComposition
            // nl.videoOutputConfiguration = self.videoSettings
            // nl.audioOutputConfiguration = self.audioSettings
            // nl.outputURL = watermarkedFilepath
            
            
            try await withCheckedThrowingContinuation { continuation in
                nl.export(completionHandler:  { result in
                    switch result {
                    case .success:
                        continuation.resume()
                    case let .failure(error):
                        continuation.resume(throwing: error)
                    }
                })
            }
            
            self.proccessedVideo = watermarkedFilepath       

       }
    }

Confirmed warnings went away with let nl = NextLevelSessionExporter(withAsset: mixComposition, qos: .userInitiated),

[piemonte]

thanks for this contribution! it has been integrated (with enhancements) and your PR is referenced in the README too.