[Metrics CLI] Create a design doc for the enabling Firebase Authentication.

[solid-dimakoniaiev]

Description
Create a feature design document for enabling Firebase Authentication based on the Analysis section of the Enabling Firebase Authentication.

Acceptance Criteria

• Add Design section to the Enabling Firebase Authentication
• Describe the solution with all relevant details including class diagrams.

Additional context
https://github.com/platform-platform/monorepo/blob/master/metrics/cli/docs/03_enablig_firebase_authentication.md