Add configurable Helm download URL for improved reliability and flexibility

[MatheusFlausino]

This PR adds the ability to configure a custom Helm download URL through the helm-download-url input parameter, providing better flexibility and reliability when the default Helm download source is unavailable.

Motivation:

• Reliability: When the default Helm download URL (https://get.helm.sh/helm-{version}-linux-amd64.tar.gz) is unavailable due to service outages or network issues, users can now specify alternative download sources
• Flexibility: Organizations with internal mirrors or specific compliance requirements can now use their preferred Helm distribution sources
• Resilience: Reduces dependency on a single download source, improving the overall reliability of CI/CD pipelines

Changes:

• Added helm-download-url input parameter to action.yml with the default value maintained for backward compatibility
• Modified installHelm() function in setup.js to accept and use the custom download URL
• Updated index.js to pass the helm-download-url input to the installHelm() function
• Updated README.md documentation to include the new parameter

Usage Example:

- name: Setup helmfile
  uses: mamezou-tech/setup-helmfile@v2.1.0
  with:
    helm-download-url: "https://your-mirror.com/helm-{version}-linux-amd64.tar.gz"

Backward Compatibility:

This change is fully backward compatible. If no custom URL is provided, the action will use the original default URL, ensuring existing workflows continue to work without modification.

Testing:

• Verified that the default behavior remains unchanged when no custom URL is provided
• Tested with custom download URLs to ensure proper functionality
• Updated documentation to reflect the new parameter

[kondoumh]

Thanks for the great PR, @MatheusFlausino!! Adding the helm-download-url input is very helpful — much appreciated.