Add AVAudioConverter bitrate strategy control

[ivanBalychev]

For current implementation at AudioCodecSettings possible to pass bitRate value, but no way to set bitRateStrategy (AVEncoderBitRateStrategyKey).

/* values for AVEncoderBitRateStrategyKey */ extern NSString *const AVAudioBitRateStrategy_Constant; extern NSString *const AVAudioBitRateStrategy_LongTermAverage; extern NSString *const AVAudioBitRateStrategy_VariableConstrained; extern NSString *const AVAudioBitRateStrategy_Variable;