feat: add cache-magento action

[damienwebdev]

PR Checklist

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• The commit message follows our guidelines: https://github.com/graycoreio/github-actions-magento2/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, the cache key for composer for Magento is non-standard, which leads to cache misses across various workflows and actions.

Fixes: N/A

What is the new behavior?

This standardizes it to a unified action to improve cache hits.

Note that I've added mode and magento_directory as inputs even though they're unused as I'd like to experiment with these in the future and adding them later would be breaking.

Does this PR introduce a breaking change?

• Yes
• No

Other information