Allow to automatically run `flutter precache`

[bartekpacia]

I often have the following code in my workflows

- name: Set up Flutter
  uses: subosito/flutter-action@v2
  with:
    flutter-version: ${{ matrix.flutter-version }}
    channel: ${{ matrix.flutter-channel }}

- name: Preload Flutter artifacts
  run: flutter precache

I wish I could so something like:

- name: Set up Flutter
  uses: subosito/flutter-action@v2
  with:
    flutter-version: ${{ matrix.flutter-version }}
    channel: ${{ matrix.flutter-channel }}
    precache: true

Possibly it should also allow for more granular selection of artifacts to precache, e.g. flutter precache --android --ios would translate to:

- name: Set up Flutter
  uses: subosito/flutter-action@v2
  with:
    flutter-version: ${{ matrix.flutter-version }}
    channel: ${{ matrix.flutter-channel }}
    precache: ['android', 'ios']