Add composer_auth support to setup-magento

[digitalrisedorset]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/mage-os/github-actions/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Currently, Composer authentication must be configured manually in workflow steps using COMPOSER_AUTH.

setup-magento does not expose a dedicated composer_auth input or propagate authentication credentials internally to Composer execution steps.

Fixes: N/A

What is the new behavior?

Adds an optional composer_auth input to setup-magento and propagates COMPOSER_AUTH to the internal Composer create-project step.

This enables authenticated Magento repository installs while preserving anonymous Mage-OS installs.

Does this PR introduce a breaking change?

• Yes
• No

Other information

This change keeps Composer authentication optional to avoid impacting existing Mage-OS workflows using the public Mage-OS mirror.

[damienwebdev]

lgtm