Base initial bandwidth estimate on first level bitrate

[robwalch]

This PR will...

Set the default bandwidth estimate based on first variant in multivariant playlist

• Adds setter for hls.bandwidthEstimate which resets the bandwidth estimator using the input value as the new default estimate.
• Adds config.abrEwmaDefaultEstimateMax option which limits how high default value can be adjusted to.

Separate peak and average bitrates of Level objects

• Level.bitrate returns peak bitrate value (BANDWIDTH attribute) by default
• Adds averageBitrate getter to Level

Why is this Pull Request needed?

This is the first of several changes that will allow selection and playback of preferred codecs on start.

Are there any points in the code the reviewer needs to double check?

This change does not change default initial quality selection. It should however provide a faster BWE ramp up, and more stable start.

Resolves issues:

Resolves #2754

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md

[robwalch]

Had to move addGroupId because it created a circular dependency between level-controller -> config -> content-steering-controller.