Add methods for extended BossBar interaction

[WasabiThumb]

This exposes the private field bossBars in order to allow checking if a FacetAudience has been shown a given BossBar, and to list the BossBars previously shown to the FacetAudience.

In my opinion, this should not be merged until and unless the main Audience interface has these methods defined in some version of Adventure, I am working on that now