Add callback to inject own statistic properties to the statistics

[JanFellner]

To be able to add own properties to the statistics i would need a callback that is called after getStats before the statistics are handled. With this approach i can inject own properties from the signalling layer e.g. how many simulcast layers are currently available or which layers have been chosen.

Current implementation i am using, but i am open to any approach offering the same: https://github.com/JanFellner/rtcstats/tree/feature/enhanced-usage

Changes: https://github.com/JanFellner/rtcstats/commit/2b4caf937f54b589a46f8e5c3f8191bfeee1a5f4