serviceCredentialsFile from secrets

[jemshit]

Hi, could you guys provide a way to use serviceAccount from secrets instead of file? As an example this action offers it

[jemshit]

For now this can be used:

- name: Decode AppDistribution ServiceAccount to JSON file
    id: decode-appdistribution-serviceaccount
    uses: timheuer/base64-to-file@master
    with:
      fileName: 'service-account.json'
      fileDir: './app_distribution/'
      encodedString: ${{ secrets.SERVICE_ACC_FIREBASE }}

then

serviceCredentialsFile: ${{ steps.decode-appdistribution-serviceaccount.outputs.filePath }}

[wzieba]

Nice idea, thanks! I'm adding this to the backlog but if anyone is willing to work on that, please feel free to provide a PR.