FR - add `commit` field for inputs

[thezackm]

The new GraphQL API includes a commit field as an optional attribute to be sent on mutations. The intention of this field is to hold the Commit SHA for a marker.

This FR is to request support for this field as an input using the default GITHUB_SHA variable:

jobs:
  newrelic:
    runs-on: ubuntu-latest
    steps:
      - name: Change Tracking Marker
        uses: newrelic/deployment-marker-action@v2.0.0
        with:
          commit: "${{ github.sha }}"
          ...

[sanderblue]

Fixed in #27