OpenWrt CI native testing for GitHub Actions

GitHub Actions doesn't allow direct inclusion of remote YAML files so we need following custom plugin which wraps the OpenWrt CI for better reusability and sharing in GitHub Actions CI environment.

Example usage

name: OpenWrt CI testing

on: [ push, pull_request ]
env:
  CI_ENABLE_UNIT_TESTING: 1
  CI_TARGET_BUILD_DEPENDS: ubus
  CI_CMAKE_EXTRA_BUILD_ARGS: -DLUAPATH=/usr/lib/lua

jobs:
  native_testing:
    name: Various native checks
    runs-on: ubuntu-20.04

    steps:
      - uses: actions/checkout@v2

      - uses: ynezz/gh-actions-openwrt-ci-native@v0.0.1

      - name: Upload build artifacts
        uses: actions/upload-artifact@v2
        if: failure()
        with:
          name: native-build-artifacts
          if-no-files-found: ignore
          path: |
            build/scan
            tests/cram/**/*.t.err