Support password managers or secret stores for local credentials

[lasomethingsomething]

User story

As a developer, I want shopware-cli to integrate with a password manager or secret store, so I do not have to copy sensitive credentials manually into commands, config files, or local setup steps.

Acceptance criteria

• secrets can be read from a supported secret source instead of plain text input
• local setup flows can consume credentials without exposing them in shell history
• documentation explains supported secret sources and fallback behavior
• manual secret entry remains possible when needed

Readiness checklist

• Acceptance criteria are clearly defined.
• Backward compatibility impact addressed.
• Documentation written.
• Tests added or adjusted accordingly.