Add GitHub client options

This release adds support for more inputs that configure the GitHub Client:

  debug:
    description: Whether to tell the GitHub client to log details of its requests
    default: false
  user-agent:
    description: An optional user-agent string
    default: actions/github-script
  previews:
    description: A comma-separated list of API previews to accept

Initial release

0.1.0

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