Fix part of #6106: Add deploy to firebase workflow

[Sandesh282]

Fixes part of #6106

Explanation

Adds the deploy_to_firebase.yml GitHub Actions workflow that allows a release coordinator to distribute a signed release AAB to Firebase App Distribution for QA testing.

This workflow:

• Accepts a gcs_aab_path input (full GCS path to a signed AAB produced by build_and_sign.yml) and an optional release_notes input
• Validates the GCS path format and extracts the flavor automatically
• Authenticates to GCP via Workload Identity Federation (same setup as build_and_sign.yml)
• Downloads the signed AAB from GCS to a temp path
• Installs the Firebase CLI and distributes the AAB using Application Default Credentials (ADC), mapping each flavor to its corresponding Firebase App ID secret
• Runs under environment: oppia-android-release-env requiring reviewer approval before execution

Essential Checklist

• The PR title starts with "Fix #bugnum: " (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• The explanation section above starts with "Fixes #bugnum: " (If this PR fixes part of an issue, use instead: "Fixes part of #bugnum: ...".)
• Any changes to scripts/assets files have their rationale included in the PR explanation.
• The PR follows the style guide.
• The PR does not contain any unnecessary code changes from Android Studio (reference).
• The PR is made from a branch that's not called "develop" and is up-to-date with "develop".
• The PR is assigned to the appropriate reviewers (reference).

[github-actions]

@Sandesh282 this PR is being marked as draft because the PR description must contain 'Fixes #' or 'Fixes part of #' for each issue the PR is changing, and each one on its own line with no other text.

[github-actions]

Coverage Report

Results

Coverage Analysis: SKIP ⏭️

This PR did not introduce any changes to Kotlin source or test files.

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