Enable use of 'Authenticate to Google Cloud' action

[adelinn]

Makes the firebaseServiceAccount optional so that GOOGLE_APPLICATION_CREDENTIALS doesn't get overwritten if the option is not defined.
Updated the readme examples to use 'Authenticate to Google Cloud' action with workload identity federation.
I didn't update the built file and the code is also not tested.

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[mvarchdev]

@adelinn Hello. What is the status? We need this urgently.

[adelinn]

I need to accept the CLA in order to allow Google to use this code, but that requires me to recreate this pull request with commits that use my real email address instead of the hidden one GitHub provides. I'm not willing to do that, so this pull request will probably remain unmerged, but I allow anyone use the code I wrote for any purpose.

You could maybe fork my branch and sync it with the main branch and run it from your fork. (No idea about legal implications of this)
Or a quick workaround is to put the contents of the credentials file the Authenticate with Google Cloud action creates into a github output and serve it to the firebase action this way.

Here is an example that makes use of workload identity federation:

      - id: auth
        name: 'Authenticate to Google Cloud'
        uses: 'google-github-actions/auth@v1'
        with:
          workload_identity_provider: 'projects/123456789/locations/global/workloadIdentityPools/my-pool/providers/my-provider'
          service_account: '[email protected]'

      - id: credential_json
        name: Prepare credentials for consumtion by firebase action
        run: cat ${{ steps.auth.outputs.credentials_file_path }} | tr '\n' ' ' | sed 's/^/value=/' >> $GITHUB_OUTPUT

      - id: firebase
        uses: FirebaseExtended/action-hosting-deploy@v0
        with:
          firebaseServiceAccount: "${{ steps.credential_json.outputs.value }}"
          repoToken: "${{ secrets.GITHUB_TOKEN }}"
          projectId: your-Firebase-project-ID
          channelId: live

[seth-acuitymd]

@adelinn @mvarchdev - FYI i recreated these commits on #319 but need to test further