Add support for custom PyPI repository configuration

[heckerdj]

Description

This PR adds the ability to configure pip to use a custom PyPI repository with authentication credentials through the setup-python action. This addresses the need for users working in enterprise environments where:

• Public PyPI is blocked by firewall
• Internal repositories (Nexus, Artifactory, etc.) are used for security-scanned packages
• Custom package indices need to be configured

Changes

• action.yml: Added three new optional inputs:

  • pypi-url: URL of the custom PyPI repository
  • pypi-username: Username for authentication
  • pypi-password: Password or token for authentication

• src/utils.ts: Implemented configurePipRepository() function that:

  • Creates pip.conf (Linux/macOS) or pip.ini (Windows) with proper configuration
  • Embeds credentials securely in the repository URL
  • Automatically masks passwords in logs using core.setSecret()
  • Handles missing or partial credentials gracefully

• src/setup-python.ts: Integrated pip configuration into the workflow, running after cache restoration but before package installation

• tests/utils.test.ts: Added comprehensive unit tests covering:

  • Configuration with URL only
  • Configuration with credentials
  • Empty URL handling
  • Partial credential warnings
  • Directory creation

• README.md: Added documentation with usage examples and security notes

Usage Example

steps:
- uses: actions/checkout@v5
- uses: actions/setup-python@v6
  with:
    python-version: '3.13'
    pypi-url: ${{ secrets.PYPI_REPO_URL }}
    pypi-username: ${{ secrets.PYPI_USER }}
    pypi-password: ${{ secrets.PYPI_PASSWORD }}
- run: pip install -r requirements.txt

Testing

All unit tests pass successfully:

• ✅ Creates pip config file with URL only
• ✅ Creates pip config file with credentials
• ✅ Does nothing when pypiUrl is not provided
• ✅ Warns when only username is provided
• ✅ Warns when only password is provided
• ✅ Creates config directory if it does not exist

Fixes #814