Authenticate with workload federation credential file instead of service account key

### The use case you're trying to solve

I would like to use a federated credential file from the auth action to configure the firebase deploy

### Change to the action that would solve that use case

```yaml
- id: auth
name: Authenticate to Google Cloud
uses: google-github-actions/auth@v0.3.1
with:
create_credentials_file: true
workload_identity_provider: ...
service_account: ${{ secrets.SERVICE_ACCOUNT_EMAIL }}

- name: 🚀 Firebase Deploy
uses: FirebaseExtended/action-hosting-deploy@v0
with:
repoToken: '${{ secrets.GITHUB_TOKEN }}'
credentials_file_path: ${{ steps.auth.output.credentials_file_path }}
projectId: '${{ secrets.PROJECT_ID }}'
channelId: live
env:
FIREBASE_CLI_PREVIEWS: hostingchannels
```