v0.3.0: Cloud Anchors and Null Safety

[CariusLars]

• BREAKING CHANGE: Converts plugin to adhere to Flutter null safety
• Adds Cloud Anchor functionality (uploading and downloading anchors to/from the Google Cloud Anchor API), including keyless authentication
• Adds Cloud Anchor example demonstrating how to use Firebase to manage cloud anchor IDs and corresponding data (e.g. on-tap texts)
• Adds data member variable to ARNode as a flexible variable to hold any information associated with the node