Add feature: audio normalization during transcoding

[YiranDuan721]

Motivation and Context

Reason for this feature: #1206
With the new feature, this problem can be avoided: #1186

Description

Applies the loudnorm filter of ffmpeg during transcoding. Parameters are chosen according to the EBU recommendation R128.
Results of some experiments with this method

Steps for Testing

Prerequisites:

• Any video(s) with one audio track
• To test locally:
  • Mock the struct StreamContext with only one member variable transcodingCmd: *exec.Cmd;
  • Mock the method StreamContext.getStreamName(void): string by returning any string (will be used in log information);
  • Mock the method StreamContext.getRecordingFileName(void): string by returning the directory of the video to test;
  • Mock the method StreamContext.getTranscodingFileName(void): string by returning the directory of the output video (with a proper file extension, such as .mp4)

1. Initialize a variable, say sc, of the mocked type StreamContext, with transcodingCmd = nil
2. Call function transcode(streamCtx: *StreamContext): error with the parameter &sc
3. Get transcoded video, of which the audio is normalized

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[alexanderstephan]

Maybe we should wait here and rebase onto https://github.com/TUM-Dev/gocast/tree/enh/runner.

[YiranDuan721]

I'll rewrite the change to runner once possible.