fix(auto-add-contributors): add check to prevent duplicate pull requests on contributor change

[sheldonhull]

Related to #75

Add a step to check for existing open pull requests using GitHub CLI before creating a new one.

• Check for existing open contributors pull request: Add a step to check for existing open contributors pull requests using GitHub CLI and exit if one already exists.
• Contribute List: Add pr_title_on_protected and commit_message input to the Contribute List step to ensure the PR title is deterministic.

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For more details, open the Copilot Workspace session.

[sheldonhull]

Update auto-update-contributors workflow to check for existing PRs

• Add step to get PR title from YAML using dockerized yq
• Add step to check for existing open contributors pull request using GitHub CLI
• Exit workflow if an existing open pull request is found
• Update pr_title_on_protected value to be deterministic

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For more details, open the Copilot Workspace session.

[sheldonhull]

Add step to check for existing open contributors pull request

• Remove step to get PR title from YAML
• Hardcode PR title in the check for existing open contributors pull request step
• Exit workflow as success if an existing pull request is found

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For more details, open the Copilot Workspace session.

[sheldonhull]

Add step to check for existing open pull requests before creating a new one

• Use gh pr list command to list open pull requests and check if a contributors pull request already exists
• Exit the workflow as a success if an existing pull request is found and comment that a pull request has already been opened
• Fix gh: To use GitHub CLI in a GitHub Actions workflow, set the GH_TOKEN environment variable. Example: env: GH_TOKEN: ${{ github.token }}

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For more details, open the Copilot Workspace session.

[sheldonhull]

Add repository checkout step to auto-update-contributors workflow

• Checkout repository: Add a step to checkout the repository using actions/checkout@v2 before checking for existing open contributors pull request.

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For more details, open the Copilot Workspace session.

[sheldonhull]

Update auto-update-contributors workflow to include commit message

• Add commit_message parameter to Contribute List step with value docs(contributor): contributors readme action update

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For more details, open the Copilot Workspace session.