Add public callbacks to help expose internal state a little more

[iandundas]

I've been using these four extra callbacks to help me keep track of WK internal state more easily:

public typealias SegmentDiscoveryCallback = (([TranscriptionSegment]) -> Void)
public typealias ModelStateCallback = ((ModelState?, ModelState) -> Void)
public typealias FractionCompletedCallback = ((Float) -> Void)
public typealias TranscriptionPhaseCallback = ((TranscriptionPhase) -> Void)

public enum TranscriptionPhase {
    case convertingAudio
    case transcribing
    case finished
} 

I just wanted to share in case they'd be useful for others. Happy to remove any that shouldn't be pulled in, or to rename them or whatever!