Migration to null saftey

• Tests running and passing on latest Dart release 2.0.5
• Migrate to null safety as detailed here: https://dart.dev/null-safety/migration-guide#step2-migrate

Ty to @afermin for the update with PR #11 🙏

Async manager initialization

Added asynchronous initialization as described here: https://docs.developers.optimizely.com/full-stack/docs/initialize-sdk-android#section-asynchronous-initialization

Non-static methods

• Marked the plugin methods as non-static to make this class easier to test/mock

Initial release

Initial release for this package. Has support for using the native Optimizely SDK's to check if a feature is enabled and fetch feature variables.