Support signing multiple images at once

[FeynmanZhou]

As a project maintainer, I have multiple images that needs to be signed at once with Notation GHA. It would be much more efficient to support signing multiple images at once by allowing input multiple image reference parameters in the Notation sign action:

- name: sign releasd artifact with signing plugin
  uses: notaryproject/notation-action/sign@v1
  with:
    ···
    target_artifact_reference: <target_artifact_reference_A>, <target_artifact_reference_B>, <target_artifact_reference_C> 
    ···