Add client package shared by fetch and push commands

Description

This PR adds a Client interface that embeds both Fetcher and Pusher. It also restructures the project to reduce code duplication.

Type of change

• Refactor

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings