Support operations on multiple stacks

[matthewmrichter]

I'd like to do a diff on a few different stacks but it is unnecessarily time consuming to run the diff (or any cdk action really) as multiple jobs, when cdk itself supports running multiple at once eg: cdk diff web api

It would be nice if the cdk_stack attribute would support multiple values. EG:

      - name: cdk diff
        uses: youyo/aws-cdk-github-actions@v2
        with:
          cdk_subcommand: "diff"
          cdk_stack:
            - web
            - api

[MPE02]

I managed to deploy multiple stacks with the solution from #48.
In your example you just would need to change the cdk_stack line to:
cdk_stack: "@(web|api)"