[Research] Authenticate users using Firebase authentication

[AlexandraDaraban]

• Closes [Research] Authenticate users using Firebase authentication #121
• Added Kotlin 1.3.21
• Created a phoneAuth activity and viewModel that should replace the login screen in the future
• Updated the flow of the login page: error handling and displaying the code input only after validating the phone number

Notes:

• google-services.json files should be added in the project; for testing purposes, you can add the configuration file in dev package.
• You need to whitelist fictional numbers in Firebase console
• On sign-in completion, a Firebase user is created with the phone number. Therefore, a call in backend should be made prior to signing in to Firebase and a call after sign-in completion to fetch the authorization token.