feat: make firebaseServiceAccount input optional

[lucetre]

Key Changes:

1. Optional firebaseServiceAccount Input:
   The firebaseServiceAccount input is no longer required. If not provided, the action defaults to using the Application Default Credentials (ADC) available in the GitHub Actions environment. This reduces the need for explicitly managing and passing a service account key JSON file.

jobs:
  demos:
    runs-on: ubuntu-latest
    permissions:
      contents: "read"
      id-token: "write"

    steps:
      - uses: actions/checkout@v4
      - uses: google-github-actions/auth@v2
        with:
          project_id: ${{ vars.PROJECT_ID }}
          workload_identity_provider: ${{ vars.WORKLOAD_IDENTITY_FEDERATION_PROVIDER_ID }}
          service_account: ${{ vars.SERVICE_ACCOUNT }}

      - uses: FirebaseExtended/action-hosting-deploy@main
        with:
          # firebaseServiceAccount: CAN_BE_OMITTED
          projectId: ${{ vars.PROJECT_ID }}
          channelId: live
          entryPoint: ./demo

Expected Outcomes

1. Improved Security:
   By leveraging default credentials, this change minimizes the usage of static key JSON files, which can pose security risks if mishandled or exposed.
2. Simplified Workflow Configuration:
   Users can now avoid creating and storing a firebaseServiceAccount secret unless explicitly needed, streamlining the setup process and reducing overhead.
3. Backward Compatibility:
   Existing workflows that supply a firebaseServiceAccount key JSON will continue to function as before, with no changes required.

[DevSusu]

LGTM

[lucetrez]

👍

[cychoi1q]

lgtm