Fixes#454
Add support for passing a list of docker images to attest.
* **action.yml**
- Add a new input parameter `subject-images` to accept a list of docker images.
- Update the `runs` section to handle the `subject-images` input.
* **src/main.ts**
- Import `parseMultiImageInput` function from `utils.ts`.
- Add logic to handle the `subject-images` input and process multiple docker images for attestation.
* **README.md**
- Update the documentation to include usage instructions for the `subject-images` input.
- Add an example for attesting multiple docker images.
* **__tests__/main.test.ts**
- Add test cases to verify the functionality of attesting multiple docker images using the `subject-images` input.
* **src/utils.ts**
- Add a new file to include the `parseMultiImageInput` function to parse the `subject-images` input.
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For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/actions/attest-build-provenance/issues/454?shareId=XXXX-XXXX-XXXX-XXXX).
* actions/attest-build-provenance/predicate from 1.0.0 to 1.1.0
* actions/attest from 1.2.0 to 1.3.0
Signed-off-by: Brian DeHamer <bdehamer@github.com>
