Support looking up snapshot

[stevehipwell]

Is your feature request related to a problem? Please describe.
In some repositories the cost of calculating the dependencies is is high enough that they're only calculated on a material change which means that not every commit has a snapshot. This causes an issue with this action.

Describe the solution you'd like
I'd like this action to support looking up the most recent snapshot for the base ref.

Describe alternatives you've considered
I'm currently using this actions pattern to lookup a valid snapshot before calling this action. This whole process would be easier if there was a REST endpoint to lookup the snapshots rather than having to compare and check for errors.

    steps:
      - name: Checkout
        uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
        with:
          fetch-depth: 0

      - name: Lookup snapshot
        id: snapshot
        env:
          GH_TOKEN: ${{ github.token }}
        run: |
          set -euo pipefail

          base_sha="${{ github.event.pull_request.base.sha }}"
          head_sha="${{ github.event.pull_request.head.sha }}"

          counter="0"
          while ((counter < 10)); do
            if [[ -z "$(gh api --header "Accept: application/vnd.github+json" --header "X-GitHub-Api-Version: 2022-11-28" --include "/repos/${{ github.repository }}/dependency-graph/compare/${base_sha}...${head_sha}" | grep -E 'X-Github-Dependency-Graph-Snapshot-Warnings:\s*[a-zA-Z0-9]+' || true)" ]]; then
              echo "::notice::Snapshot found on SHA ${base_sha}"
              {
                echo "base_sha=${base_sha}"
                echo "head_sha=${head_sha}"
              } >> "${GITHUB_OUTPUT}"
              exit 0
            fi

            base_sha="$(git rev-parse "${base_sha}~1")"
            counter=$((counter+1))
          done

          echo "::error::Snapshot not found"
          exit 1

      - name: Dependency Review
        uses: actions/dependency-review-action@595b5aeba73380359d98a5e087f648dbb0edce1b # v4.7.3
        with:
          base-ref: ${{ steps.snapshot.outputs.base_sha }}
          head-ref: ${{ steps.snapshot.outputs.head_sha }}
          fail-on-severity: high
          comment-summary-in-pr: always

Additional context
N/A