Add a boolean output to indicate if the image was pushed

[Clockwork-Muse]

Please add an output that indicates whether or not the image was successfully pushed.
This can be useful in cases where the push may be filtered out, and rather than repeating the same filter on a subsequent step it would be simpler to condition it based on the output from this action. For example:

- id: build
  name: Pre-build dev container image
  uses: devcontainers/ci@v0.3
  with:
    imageName: someRepo/someImage
    imageTag: someTag
    push: filter
    refFilterForPush: |
      refs/heads/main

- id: get-digest
  name: Get image Digest
  run: |
    digest=$(docker inspect --format={{.Id}} someRepo/someImage:someTag
    echo "Image Digest: $digest"
    echo "digest=${digest}" >> $GITHUB_OUTPUT

- id: attest
  name: Attest
  uses: actions/attest-build-provenance@v3
  with:
    subject-name:  someRepo/someImage
    subject-digest: ${{ steps.get-digest.outputs.digest }}
    push-to-registry: ${{ steps.build.outputs.wasPushed }}