v3.0.0 - Null Safety

• Migrate to null safety
• Minimum Dart SDK version increased to 2.12.0
• Minimum Flutter SDK version increased to 1.20.0

API changes:

• Voximplant.getAudioDeviceManager() -> Voximplant.audioDeviceManager
• Voximplant.getCameraManager() -> Voximplant.cameraManager
• Voximplant.getMessenger() -> Voximplant.messenger
• Added named settings argument to VIClient.call(), VIClient.conference() and VICall.answer()

Other changes:

• VIConversation.title field nullability changed to nullable
• VIConversationConfig.title field nullability changed to nullable
• VIAuthResult.loginTokens field nullability changed to nullable
• Minor internal improvements