Service account json support added and README updated

[chibombo]

 What's new

• Service account JSON support
• README updated
• Github builds the repository

Details

Firebase deprecated the TOKEN implementation and the README contained the instruction for the deprecated implementation. This PR updates:

• Google URLs
• The user can use the deprecated implementation but we need to generate the FirebaseSDK private key manually in the Firebase Console
• Github Actions builds the project in the latest Ubuntu version available. PRs trigger it.