Add PreferredAudioLabel Configuration

[tyrelltle]

I have submitted a PR #4763 for this issue

Have you read the FAQ and checked for duplicate open issues?
Yes

Is your feature request related to a problem? Please describe.
The DASH manifest used by our shaka integration may have different audio "label" in each adaptation set. Its different from audio "language".
Based on user configuration, we need the player start with a variant that has specific audio label.

• The requirement is that, shaka should select the variant with right label, at very beginning, instead of sometime by calling selectVariantByLabel

Currently there is no way to configure default label in shaka.

Describe the solution you'd like
AdaptationSetCriteria already supports label filtering, but player configuration does not have option for it (it only has preferredAudioLanguage).
The solution is to add preferredAudioLabel to player configuration.

Describe alternatives you've considered
We tried alternative approach which is listen to "streaming" event from shaka, and call selectVariantByLabel at that time.
However in 4.3.x version, this no longer works because player.js initializes this.playhead_ after "streaming" event, while selectVariantByLabel requires this.playhead_ to be set in advance.

Additional context

[tyrelltle]

I have submitted a PR for it:
#4763