[WIP] Monobehaviour refactoring

[StephenHodgson]

• closes Non MonoBehaviour implementation of Purchases.cs #612

Overview

This is a major breaking change refactor, that decouples the Monobehaviour requirements for the Purchases SDK.

• Default RevenueCat namespace was added to all types.
  • Moved all types out of nested Purchases partial class into their own classes under the RevenueCat namespace
  • Refactored the data models to use Newtonsoft.Json serialization (using the built-in unity package)
• Added a RevenueCatConfig scriptable object for developers to be able to configure the SDK settings in the new RevenueCat player projects settings window (or directly on the config.asset itself).
• Added PurchashesSDK static class that is the central point of contact for developers to interact with the SDK
• Refactored the native plugin wrappers and removed the Unity Send Event Implementations, and standardized the native interop and ensuring that all async tasks are returned to the unity main thread context.
• Fixed missing dependency references to google external dependency resolver for automatic resolution when installing from OpenUPM

TODO

• macOS/iOS/visonOS native wrapper refactor
• add samples and demo scenes for The demo app isn't enough to represent a real-world scenario #643
• update integration tests and CI/CD
• migration guide and docs