feat: add Expo config plugin for mParticle integration and create Expo test app

- Introduced a new Expo config plugin to facilitate mParticle SDK integration for both iOS and Android.
- Created an Expo test app to demonstrate the usage of the mParticle SDK, including event logging and Rokt placements.
- Updated onboarding documentation to include instructions for using the new plugin and test app.

Author: thomson-t

ExpoTestApp/.gitignore

@@ -0,0 +1,27 @@
+# Dependencies
+node_modules/
+
+# Expo
+.expo/
+dist/
+web-build/
+
+# Native builds
+ios/
+android/
+
+# Debug
+npm-debug.*
+yarn-debug.*
+yarn-error.*
+
+# macOS
+.DS_Store
+*.pem
+
+# Local env files
+.env*.local
+
+# TypeScript
+*.tsbuildinfo
+