Return secrets as outputs instead of values

[Olfi01]

Issue #14: Environment variables vs outputs

Description of changes:
Secrets are now returned as action outputs. This results in the following improvements:

• User side:
  • Less concern needed over which steps to separate in a different job to avoid access to environment variables
  • Full control over where the returned secrets are used
• Codebase side:
  • No more need to clean up environment variable secrets, as the GitHub Actions toolkit will handle the outputs.

README.md, action.yml and code comments have been updated accordingly. Tests are passing.

If you prefer reviewing less files and compiling the source code yourselves, I have created a parallel PR(#37) containing only the code changes without the built dist files.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[Olfi01]

@ecraw-amzn Is there something I still need to do before this PR can be reviewed and merged?

[jbct]

Thanks for the PR, @Olfi01 - we are evaluating this as a part of issue #14

[tvb]

@jbct can we merge this. Currently, we are facing issues due to the Secrets Cleanup right after the fetch.
Nevermind, the documentation wasn't clear we shouldn't use vars., but use env. instead to access the variables.

[Danny-Smart]

Is there anything holding this feature up?