feat: impl cache hit outputs

[anies1212]

What does this PR do?

This PR adds a new output, cache-hit, to the flutter-action. The cache-hit output indicates whether the cache was successfully hit during the setup process. This can help optimize workflows by conditionally executing subsequent steps based on cache status.

Why is this change necessary?

In our project, we manage a monorepo using melos. As part of our CI/CD pipeline, we run melos bs (bootstrap) every time to set up dependencies and link packages. However, this can be redundant when the cache is already valid.

With the cache-hit output, we can:

• Avoid unnecessary executions of melos bs when the cache is valid.
• Reduce build times and resource usage.
• Enhance workflow efficiency.

Example usage

Here’s how we plan to use the cache-hit output:

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout repository
        uses: actions/checkout@v4

      - name: Set up Flutter
        uses: subosito/flutter-action@v2
        id: flutter-action
        with:
          channel: stable
          cache: true

      - name: Conditionally run melos bootstrap
        if: steps.flutter-action.outputs.CACHE-HIT == 'false'
        run: melos bs

      - name: Continue with build
        run: flutter build apk