Add platform argument for container image scans

[bluesentinelsec]

This PR adds a new argument for container image scans: platform.

Platform is used to indicate the OS and CPU architecture of the image you wish to scan.

Platform must be specified in the form, os/cpu/variant, where variant is optional but os/cpu are required.
For example: linux/amd64, linux/arm64/v8.

Windows container images are not supported at this time.

This workflow requires inspector-sbomgen versions greater than 1.5.1.

Example workflow snippet:

      - name: Test container scan
        id: inspector
        uses: aws-actions/vulnerability-scan-github-action-for-amazon-inspector@v1
        with:
          artifact_type: 'container'
          artifact_path: 'ubuntu:14.04'
          platform: "linux/arm64"
          display_vulnerability_findings: "enabled"
          sbomgen_version: "latest"

This PR addresses issue #101