Support reading Flutter version from config file (FVM)

[kuhnroyal]

I want to bring this up for consideration. FVM (https://github.com/leoafarias/fvm) is a pretty popular version manager for Flutter. It would be great if the action would support reading the Flutter version/channel from its config file in .fvm/fvm_config.json similar to the ruby action and others.

Currently I am reading the version manually from the JSON file in every pipeline:

      - name: Determine Flutter version
        uses: antifree/json-to-variables@v1.0.1
        with:
          filename: '.fvm/fvm_config.json'
          prefix: fvm
      - uses: subosito/flutter-action@v1
        with:
          flutter-version: ${{ env.fvm_flutterSdkVersion }}

This could just be something like this:

    - uses: subosito/flutter-action@v1
      with:
        flutter-version: '.fvm/fvm_config.json'

I would love to create a PR if there is interest for this.

[kuhnroyal]

I created a simple action that parses the FVM config file. It can be used together with flutter-action as well as with custom setup.

https://github.com/kuhnroyal/flutter-fvm-config-action