Automatic installation of GitHub CLI, Git and Git LFS

[user21t]

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I have added requests and urllib3 packages in the first place to handle downloading files from Internet via Python script. Then I have rewritten code in pbsync/main.py to allow PBSync for automatic installation of GitHub CLI, Microsoft's Git and Git LFS. For more information see ticket #7.

First, the program checks whether GitHub CLI isn't already installed. If yes, then it proceeds forward. Otherwise, it starts downloading installation file for GH CLI. When downloading fails once, the program will repeat the process 5 times at most until passing. If download and installation process is successful, then Git and Git LFS are installed via GitHub CLI and terminal commands.

Workflow is improved, because we don't download separately by a browser. We can also repeat the version check/install process. Since we are repeating GitHub CLI download process, we don't have to terminate it because of temporary problems like no Internet connection.