Unified Backups: Migrate and refactor repo target

What does this MR do and why?

This merge request updates the GitLab Backup CLI, a command-line tool for backing up and restoring GitLab data. The main changes include:

• Adds support for backing up and restoring repositories using Gitaly.
• Decouples with RepoType from the rails codebase. Introduces a new RepoType class to represent different types of repositories (e.g., project, wiki, snippet, design).
• Updates the Targets module to include a new GitalyBackup class for backing up and restoring repositories using Gitaly.
• Updates the Tasks module to include a new Repositories task for backing up and restoring repositories using Gitaly.
• Makes various improvements to the codebase, including adding tests, refactoring, and improving documentation.

Related issue: #481370 (closed)

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