test: add e2e tests setup

[pkaramon]

Summary

Added setup for writing e2e using Maestro framework:

• added testIds for controls in the example app
• added button for "Test screen", so we can customize the layout of the example app for tests
• changed toolbar to a grid in "Test screen"
• documented testing in CONTRIBUTING.md
• added to new scripts test:e2e:android, test:e2e:ios
• changed bundle identifier to swmansion.enriched.example, so it's the same as applicationId on android
• added example tests

Test Plan

1. Install maestro
2. Run example app on specified devices
3. Run all tests on android, ios; run individual test.
4. Test UPDATE_SCREENSHOTS flag, by first breaking an existing test, and then setting it to true.

Screenshots / Videos

Added test button:

Compatibility

OS	Implemented
iOS	✅
Android	✅