[Feature request]: Voice Guidance Event Callbacks

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Use case

When building navigation apps with custom voice announcements (e.g., toll section warnings, speed alerts), there's no way to coordinate timing with the SDK's built-in voice guidance. This causes overlapping speech where neither message is audible, resulting in a poor user experience.

Currently, we can only estimate when the SDK might be speaking based on `NavInfo.distanceToNextStepMeters`, but this is unreliable since:
- Announcement timing varies by maneuver type and road conditions
- Traffic alerts and rerouting announcements are unpredictable
- Different languages have different speech durations

### Proposal

Add event callbacks to notify when voice guidance speech starts and completes:
```
// Option A: Dedicated listener
GoogleMapsNavigator.setVoiceGuidanceListener((VoiceGuidanceEvent event) {
switch (event) {
case VoiceGuidanceStarted(instruction: final text):
// SDK started speaking - pause custom TTS queue
case VoiceGuidanceCompleted():
// SDK finished speaking - resume custom TTS queue
}
});

// Option B: Property on NavigationAudioGuidanceSettings
GoogleMapsNavigator.setAudioGuidance(
NavigationAudioGuidanceSettings(
guidanceType: NavigationAudioGuidanceType.alertsAndGuidance,
onSpeechStart: (String instruction) => ...,
onSpeechComplete: () => ...,
),
);
```