github-actions: support schedule cron

[pjfanning]

Github Actions yml supports setting a schedule param. It would be nice to have sbt-typelevel-github-actions support an sbt param that allows users to set this.

on:
  push:
    branches: [ master ]
  pull_request:
    branches: [ master ]
  schedule:
    - cron: '33 9 * * 4'

Example: https://github.com/apache/commons-crypto/blob/master/.github/workflows/codeql-analysis.yml

[armanbilge]

Thanks for the issue! An additional sbt setting to support this sounds good to me 👍

Long term I would like to better formalize support for GHA workflows with a more complete model based on the specification. Currently there are too many gaps like this.