[Backend] Add GitRepository lib class

[zergov]

Part of: https://github.com/visevol/GihubVisualisation/issues/10

Add the GitRepository class in the lib directory.

This is a wrapper over the process of cloning and extracting logs from a git repository.

How to test locally

1. Build the api docker container: docker compose build api
2. Open a rails command line: docker compose run api bin/rails c
3. From this console, you can instantiate the GitRepository class of this PR and play with it:

   repo = GitRepository.new("https://github.com/rails/rails")
   repo.clone
   repo.logs do |logs|
   logs.each do |line|
   p line
   end
   end

^ This clones the https://github.com/rails/rails repository, and dumps the content of all its commits (93 000).